Robert J. Costello, Davidoff Hutcher & Citron.

For the past two years, Special Counsel Robert Mueller's investigation into Russian interference in the 2016 presidential election has spotlighted the work of several defense lawyers for President Donald Trump and his former associates.

But Mueller's work, culminating in the release of a redacted report Thursday, has only recently highlighted the activities of New York attorney Robert Costello, who worked behind the scenes on behalf of Michael Cohen, Donald Trump's former fixer and lawyer.

Costello, a partner at Davidoff Hutcher & Citron, is featured several times in Mueller's report, which explains Costello's diligent attempts to connect Cohen with Rudy Giuliani, Trump's lawyer. Costello, according to the report, told Cohen that he had a "back channel of communication" to Giuliani and that Giuliani had said the "channel" was "crucial" and "must be maintained."

Costello has handled both criminal and civil cases, according to his firm's website. He has represented famous clients before, including Leona Helmsley, the businesswoman dubbed the "Queen of Mean," and George Steinbrenner, the longtime owner of the New York Yankees who fought for years in the 1980s and 1990s to have his ban from managing a baseball team overturned. Besides litigation, Costello has also handled administrative law and municipal investigations, his firm website said.

Costello is described in the Mueller report as having a "close relationship" with Giuliani. The Davidoff Hutcher partner worked in the U.S. Attorney's Office for the Southern District of New York for several years until 1981, according to his LinkedIn profile. It was around that time Guliani came to prominence in the city where he was later elected mayor, becoming U.S. Attorney of the Southern District in 1983.

The New York Law Journal reported in 1993, during Giuliani's successful run for mayor, that Costello was one of his top fundraisers. It was reported at the time that he "became friendly with Mr. Giuliani after spending a summer in the early 1970s working for him as a law student intern at the U.S. Attorney's office."

According to Mueller's report, Costello was in contact with Cohen after Cohen's home and offices were searched by federal investigators on April 9, 2018. Cohen was put in touch with Costello by Jeffrey Citron, a managing partner at Davidoff Hutcher & Citron described by The New York Times as an acquaintance of Cohen's, on April 17. A few days later, on the same day that Trump criticized a Times report about the possibility of Cohen "flipping," Costello emailed Cohen that he had spoken with Giuliani and said the conversation was "very very positive."

"You are 'loved' ... they are in our corner," Costello told Cohen in one email cited in Mueller's report. "Sleep well tonight, you have friends in high places."

A footnote in the Mueller report citing that email includes a line that is redacted, citing the risk of "harm to ongoing matter." The New York Times reported last month that federal prosecutors have "requested" Costello's emails and documents related to Cohen, but it is not clear what they are investigating or if the redaction was related to that reported request.

More of Costello's emails with Cohen were published Sunday in another New York Times report, which says Costello was never formally retained by Cohen. (Tom Butler, a Davidoff Hutcher & Citron spokesman, said Cohen had a copy of a retainer agreement and wrote that the firm was "on his team.") The article depicts Costello struggling to convince Cohen to stick to the strategy of staying in Trump's good graces — and eventually failing.

“You are under the impression that Trump and Giuliani are trying to discredit you," Costello said in an email to Cohen in mid-June, cited by Times. “I think you are wrong because you are believing the narrative promoted by the left-wing media.”

Mueller's report said Costello's firm billed Cohen for its work July 7, 2018. The Times reported that Costello bowed out from working for Cohen around that time.

Cohen eventually retained Guy Petrillo of Petrillo Klein & Boxer to negotiate a deal with prosecutors. Cohen had also hired Stephen Ryan of McDermott Will & Emery, who represented Cohen in his challenge to the government's review of the files it seized from his home and office.

Butler said Costello was on trial Monday and could not be interviewed. Giuliani did not return a message seeking comment Monday.

Despite Costello's reportedly close relationship with Giuliani, his firm has been adverse to Trump and his real estate empire multiple times, said Larry Hutcher, another managing partner at Davidoff Hutcher, in an interview.

In one such case filed last year, the firm represented Trump's longtime driver Noel Cintron, who said he was underpaid during the years of service he provided the Trump Organization. Cintron said he had to be available at the drop of a hat and often worked overtime that he wasn't paid for. Hutcher said that case is currently in arbitration.