"It never ceases to amaze me how successful you have been making yourself so small, petty, and banal with your tweets," Mr. Brennan told the president on Twitter. "Your insecurity is well deserved, as is your concern over Russia investigation."
The ex-CIA chief concluded, "Thomas Paine was right when he said, "These are the times that try men’s souls."
He was responding to Mr. Trump’s tweet earlier Tuesday in which the president said, "I have been much tougher on Russia than Obama, just look at the facts. Total Fake News!"
Mr. Brennan led the CIA from 2013 to January 2017, when Mr. Trump became president. Mr. Trump has stepped up his comments online about Russia since special counsel Robert Mueller won indictments on Friday against 13 Russian individuals and three Russian groups for meddling in the 2016 election.
Mr. Trump has repeatedly questioned why the Obama administration didn’t do more to stop the election interference.