It was March 2018, nearly 10 months into his Russia investigation, when special counsel Robert Mueller III, a man of few words, raised the stakes dramatically in a meeting with Donald Trump's lawyers: If the president did not sit down voluntarily for an interview, he could face a subpoena.
In the months that followed, Mr Mueller never explicitly threatened to issue a subpoena as his office pursued a presidential interview, a sit-down for which the special counsel was pushing as late as December.
But with that prospect hanging over them, Mr Trump's legal team conducted a quiet, multi-pronged pressure campaign to avert such an action and keep the president from coming face-to-face with federal investigators - fearful he would perjure himself.
At one point last summer, when a lull in talks had the president's attorneys worried that Mr Mueller was seriously contemplating a subpoena, White House lawyer Emmet Flood wrote a memo laying out the legal arguments for protecting the president's executive privilege. He sent the document to Mr Mueller's office and to the deputy for top Justice Department official Rod Rosenstein, who oversaw the probe, according to two people familiar with Mr Flood's outreach.
Meanwhile, the Trump lawyers sent a steady stream of documents and witnesses to the special counsel, chipping away at Mr Mueller's justification for needing an interview with the president.
In the end, the decision not to subpoena the president is one of the lingering mysteries of Mr Mueller's 22-month investigation, which concluded last week when he filed a report numbering more than 300 pages.
The special counsel did not find a conspiracy between the Trump campaign and Russia, but - in an unusual move - failed to come to a decision about whether Mr Trump obstructed justice, according to a summary of the Mueller report released by attorney general William Barr. An interview with the president would have been pivotal to helping assess whether the president had corrupt intent, a key element of such a charge, legal experts said.
It is an open question whether a subpoena would have survived the court challenge Mr Trump's lawyers say they would have waged. The Supreme Court has never issued definitive guidance on issuing a subpoena to a president, but had Mr Mueller pursued one, the courts could have established a precedent for future presidents.
In assessing whether to pursue such a high-stakes move, the special counsel was not operating with complete autonomy. That was a contrast with predecessors such as Kenneth Starr, who investigated President Bill Clinton and had broad leeway under the now-expired independent counsel statute.
But Mr Mueller was supervised by Mr Rosenstein, a Trump appointee. The special counsel, Mr Rosenstein noted in one letter to a Republican senator, "remains accountable like every other subordinate."
Mr Rosenstein himself was under intense political pressure: Mr Trump mused about firing the one-time George W Bush appointee and former US attorney for Maryland, whom he derided at one point as "the Democrat from Baltimore." And House conservatives threatened to impeach Mr Rosenstein, accusing him of withholding information about the Russia probe.
Internal Justice Department discussions about whether to subpoena the president - including Mr Rosenstein's views on such an action - remain tightly held.
In the final months of the probe, there was upheaval in the department's leadership. Mr Trump ousted attorney general Jeff Sessions, who had recused himself from the investigation. He was replaced temporarily by his former chief of staff, Matthew Whitaker, who was publicly critical of the special counsel before joining the department.
A month before Mr Mueller submitted his report, Mr Barr was confirmed as attorney general. He had questioned Mr Mueller's obstruction-of-justice inquiry in a June 2018 memo to Mr Rosenstein months before his appointment, writing that "Mueller should not be permitted to demand that the President submit to interrogation about alleged obstruction."
If Mr Mueller wanted to push for a subpoena, he did not force the issue with Justice Department leaders. Mr Barr told lawmakers last week that no decision the special counsel wanted to take was vetoed during the investigation.
The Justice Department and the special counsel's office declined to comment.
More answers could be revealed in Mr Mueller's full report, which House Democrats are pushing Mr Barr to release.
What is known is that the president's lawyers now believe keeping their client from sitting down with investigators was their greatest victory.
"The president would not have helped his case had he gone in," said Mark Corallo, a former spokesperson for Mr Trump's legal team. "No lawyer worth his salt would let that happen."
The president was initially inclined to sit for an interview with Mr Mueller. He thought he could deliver a convincing performance and put a swift end to the probe.
Negotiations between the sides began around Thanksgiving 2017, and an interview was scheduled for January 2018, according to a person close to the legal team and a former senior administration official.
But John Dowd, then the president's lead attorney, cancelled the session. He had argued against it because he feared Mr Trump could misspeak or even lie. And a practice session with the president further convinced Mr Dowd that the president could be a problematic interviewee, these people said.
White House officials declined to comment.
Over the next 12 months, Mr Mueller tried repeatedly to reschedule the interview, to no avail.
Mr Trump continued to state publicly that he would be glad to sit for an interview - he believed being seen as willing to talk with prosecutors showed "strength," according to a former administration official with direct knowledge of his thinking. But the president came to agree with his lawyers that doing so would be too risky, especially after former national security adviser Michael Flynn pleaded guilty to lying to the FBI in December 2017, current and former White House aides said.
Trump lawyer Rudy Giuliani said that some of what Mr Trump's legal advisers were hearing from Mr Mueller "raised our suspicion that this is a trap, rather than a search for more information."
As the standoff continued, Mr Mueller's team discussed at length the idea of issuing a subpoena, if necessary, to compel Mr Trump to sit for an interview, according to a person familiar with the matter who spoke on the condition of anonymity to describe internal conversations.
The discussions - which included Mr Mueller, his top deputy James Quarles, and prosecutors Michael Dreeben and Aaron Zebley - centred both on whether a subpoena was legally feasible and what the costs of such a move might be to the overall investigation, the person said.
A fight over a presidential subpoena would have been likely to set legal precedent.
Under President Richard Nixon, the US Supreme Court ruled that investigators could subpoena evidence from a sitting president and ordered Nixon to turn over materials including secret recordings made in the Oval Office. That ruling did not, however, address testimony by the president.
When Mr Starr was independent counsel, he issued a subpoena to Mr Clinton ordering the president to testify before a grand jury about his relationship with White House intern Monica Lewinsky. Mr Clinton's team considered challenging the subpoena in court but instead decided that it would be politically damaging to be seen as fighting the investigation. Mr Clinton's lawyers agreed that he would voluntarily sit for an interview, and Mr Starr withdrew the subpoena - leaving open the question of whether a president can be compelled to give testimony.
Robert Ray, a former independent counsel now in private practice at Thompson & Knight, said Mr Mueller's team would have had to weigh whether a subpoena could survive the court challenge that was all but certain to come from the Trump White House.
The Supreme Court has never issued definitive guidance on the question, but in a previous independent counsel investigation, of Mike Espy, an agriculture secretary in the Clinton administration, an appellate court offered some clarity on the bounds of how the White House could fight a subpoena by citing presidential privilege.
On the basis of the precedent from that case - which was focused on documents, rather than an interview – Mr Mueller would have had to demonstrate both a need to subpoena Mr Trump to advance his investigation and show that he could not get the information he sought in any other way, Mr Ray said.
Another major factor was time: Mr Mueller had to consider the likelihood that such a move would bog the investigation down in a lengthy legal battle.
"That's a major fight, and you have to decide whether, in the country's best interests, it's worth it," Mr Ray said.
Mr Mueller broached the topic during a tense meeting on 5 March 2018, at the special counsel offices in Southwest Washington, as Mr Trump's attorneys maintained that the president had no obligation to talk to investigators.
The special counsel noted there was an option if Mr Trump declined: He could be subpoenaed to appear before a grand jury, as The Washington Post previously reported.
Mr Dowd erupted angrily.
"You're screwing with the work of the president of the United States," he told Mr Mueller, according to two people briefed on the discussion.
After that meeting, the special counsel team changed its approach: trying to coax Mr Trump to sit for an interview voluntarily.
Prosecutors hoped the president would agree to meet, mindful that they could not explicitly threaten a subpoena unless they were prepared to issue one, according to a person familiar with the matter.
Still, Mr Trump's legal advisers felt after the March meeting that a subpoena threat hung over the president.
"The whole exercise was premised on the idea that that was a legal option they could pursue, and we were never absolutely sure until the end that they would not," said one Trump adviser familiar with the legal negotiations.
That threat governed the president's legal strategy in the months that would follow.
Mr Trump's lawyers left the distinct public impression that they were not an equal match for Mr Mueller, a venerated former FBI director. Mr Dowd and Ty Cobb, another legal adviser to Mr Trump, were overheard by a reporter discussing over lunch at a popular Washington steakhouse how much they would cooperate with Mr Mueller. Mr Giuliani developed a habit of misspeaking in meandering television interviews.
But behind the scenes, Mr Trump's legal advisers had a quiet weapon: a husband-and-wife pair of criminal lawyers, Jane and Martin Raskin, who brought rigor and regimen to the team when they came aboard in April 2018.
While Mr Giuliani and attorney Jay Sekulow managed the public relations strategy, the Raskins did most of the lawyering from a temporary office they set up in Washington. They declined to comment.
Mr Giuliani said that roughly 80 per cent of the Trump team's interactions with the special counsel's office were handled by Jane Raskin, who has known both Mr Mueller and Mr Quarles for years. She knew Mr Mueller from her time as a federal prosecutor in Boston, while her husband had worked with Mr Quarles.
She communicated mostly by email, developing a written record that Mr Trump's attorneys intended to use as evidence of their cooperation and responsiveness if they ended up in court fighting a subpoena.
Martin Raskin, meanwhile, did a great deal of the writing and editing of legal arguments, including a "counter report" defending the president that Mr Giuliani said has been prepared but may never be released.
Central to the Trump strategy - developed first by Mr Cobb and Mr Dowd and later carried out by Mr Giuliani, Mr Sekulow and the Raskins, as well as Mr Flood, who from his White House perch represented the office of the presidency - was to cooperate fully with every request for documents and witnesses from Mr Mueller, including Mr Trump's written answers to some questions.
Their goal: to satisfy Mr Mueller's hunt for information to the extent that the special counsel would not have legal standing to subpoena the president's oral testimony.
"We allowed them to question everybody, and they turned over every document they were asked for: 1.4 million documents," Mr Giuliani said. "We had what you would call unprecedented cooperation."
Mr Trump's lawyers, citing the independent counsel investigation of Mr Espy, argued that to justify a subpoena of Mr Trump, Mr Mueller needed to prove he could not get the information in any way other than by asking the president.
"No matter what question they would say they wanted to ask, I felt confident we could turn it over and say, 'You already have the answer to it,'" Mr Giuliani said. "If they said, 'Why did you fire Comey?' I'd give them five interviews, and particularly the Lester Holt tape, where he goes into great detail as to his reasons."
Mr Giuliani was referring to Mr Trump's May 2017 interview with the NBC Nightly News anchor in which the president said he was thinking about "this Russia thing" when he fired James Comey as FBI director, one of the actions Mr Mueller was investigating as possible obstruction of justice.
All the while, Mr Giuliani said, the legal team was not convinced that it would have prevailed in court. "Honestly, I don't know who would have won," he said. "I think our argument got better as time went on. But I don't know if we would have won."
As Mr Mueller's lawyers quietly laboured, a political storm was raging around them.
Mr Trump, his lawyers and his allies in Congress routinely attacked Mr Mueller and his investigators as compromised and corrupt. The president repeatedly urged an end to the probe, which he condemned as a "witch hunt," a "fraud" and a "hoax" that was wasting taxpayer money.
Mr Rosenstein urged lawmakers to respect the confidential work of the special counsel, saying in a June 2018 letter to senator Charles Grassley, R-Iowa, then the chairman of the Senate Judiciary Committee, that the probe would comply with all laws and Justice Department policies.
But Mr Rosenstein also noted that Mr Mueller was not an entirely independent actor - and that his work was being closely supervised.
"Under the terms of his appointment, both by statute and by regulation, special counsel Mueller remains accountable like every other subordinate Department official," Mr Rosenstein wrote.
A few months later, Mr Flood sent his memo on the scope of executive privilege. While it made broad arguments, the document could have been construed to pertain to Mr Mueller's push to interview the president, according to someone with knowledge of the contents.
Notably, Mr Flood sent the memo not just to Mr Mueller's office, but also to Mr Rosenstein by way of his top deputy, Edward O'Callaghan.
Mr Flood declined to comment.
As each month passed without a subpoena, the president's attorneys increasingly doubted that Mr Mueller would seek to obtain one, according to people with knowledge of internal discussions.
Mr Mueller's team kept insisting it needed to interview the president - but never followed through with an actual demand.
Mr Mueller and Mr Quarles would stress that they needed to know Mr Trump's intentions when he fired Mr Comey and took other actions that could have thwarted the Russia investigation. Jane Raskin would respond by pressing them for a legal justification for seeking to interview the president, according to a person familiar with the negotiations.
The president's team asked, "What evidence have you obtained that justifies you interviewing the president?" according the person, who added that Mr Mueller's office was "never able to articulate a compelling case. They never gave up asking, but they had no good answer for that question."
In the absence of an interview, Mr Trump's attorneys offered Mr Mueller a substitute: The president would provide answers to a set of questions about Russia and the campaign, submitted in writing. But, citing executive privilege, they refused to provide answers to questions pertaining to the president's time in office - questions that went to the heart of the special counsel's inquiry into possible obstruction of justice.
However, the process of compiling answers dragged. Mr Trump's lawyers found it difficult to get the president to focus on drafting the submission, according to people familiar with the sessions. Mr Trump's meetings with his lawyers were frequently interrupted by phone calls and other White House business.
Finally, in late November 2018, the lawyers sent Mr Trump's answers to Mr Mueller.
In December, Mr Mueller's team made one more request for an interview with the president.
And in January, the special counsel's office contacted Mr Trump's lawyers to ask some follow-up questions, according to people familiar with the request.
But Mr Trump's lawyers again declined. They neither agreed to an interview nor answered the additional questions.
Two months later, Mr Mueller submitted his report without having spoken to the president. The investigation was over.
The Washington Post