Newsweek had previously reported that he planned to write a newspaper op-ed.
Now that he is a private citizen, Comey can address the still murky connections between the New York FBI office, elements of the New York Police Department and Donald Trump.
James Comey's attack on the president couldn't match Stormy Daniels's recent interview but did at least have one notable viewer.
Ninety-one-year-old John Dingell said he hadn't planned on buying a copy.
The former FBI director said sending the letter had been the right thing to do.
Instead, they should get to the polls and vote Trump out of office, the former FBI director said.
Representative Jim Jordan previously claimed Hillary Clinton's campaign colluded with Russia in the 2016 presidential election.
The Justice Department inspector general began a review of Comey and McCabe in 2017.
"Donald Trump's presidency threatens much of what is good in this nation," the ex-FBI chief writes.
Blame the Russia investigation, Trump said.