Mr Comey also suggested the head of the US justice department – who has launched a review into the origins of the 2016 election meddling probe – had been acting as a “spokesperson” for Donald Trump.
“The AG should stop sliming his own Department,” the former FBI chief tweeted. “If there are bad facts, show us, or search for them professionally and then tell us what you found. An AG must act like the leader of the Department of Justice, an organization based on truth. Donald Trump has enough spokespeople.”
The attorney general has asked John Durham, the US attorney in Connecticut, to examine how the probe into Russian election interference began and whether laws were broken while intelligence was collected on the Trump campaign, it was revealed earlier this week.
On Friday Mr Barr said the review would focus on the actions of the US intelligence community before the FBI opened a formal inquiry in July 2016.
“Government power was used to spy on American citizens,” the attorney general told The Wall Street Journal. “I can’t imagine any world where we wouldn’t take a look and make sure that was done properly.”
Mr Barr told a Senate subcommittee last month that he believed “spying did occur”. He said: “The question is whether it was adequately predicated and I’m not suggesting it wasn’t adequately predicated, but I need to explore that.”
The attorney general has provided no details about what “spying” may have taken place but he could be alluding to a surveillance warrant the FBI obtained on former Trump associate Carter Page and the FBI’s use of an informant while investigating ex-Trump campaign foreign policy adviser George Papadopoulos.
The AG should stop sliming his own Department. If there are bad facts, show us, or search for them professionally and then tell us what you found. An AG must act like the leader of the Department of Justice, an organization based on truth. Donald Trump has enough spokespeople.— James Comey (@Comey)May 18, 2019
His suggestions that members of the Trump campaign were unfairly targeted have been welcomed by the president and his associates, who have repeatedly claimed investigations into the campaign were motivated by political bias.
The president said that he did not request Mr Barr launch the review, but that he thinks “it’s a great thing that he did it.”
FBI Director Christopher Wray told Congress earlier in May that he has no evidence the FBI illegally monitored the Trump campaign and doesn’t consider court-approved FBI surveillance to be “spying”.
Mr Comey has said “the FBI doesn’t spy, the FBI investigates”. He has been a consistent critic of Mr Trump since he was fired as FBI director by the president in May 2017, calling him “morally unfit to be president”.