FBI agent Peter Strzok was removed last summer from Mueller's investigation team after it was found that he had texted anti-Trump messages with another FBI agent.
Strzok was once a key figure in the 2016 FBI investigation into former Secretary of State Hillary Clinton's handling of classified material.
He was removed from the special counsel investigation into Russia's election meddling after it was discovered he exchanged anti-Trump text messages during the investigation with fellow FBI agent Lisa Page, with whom he was reportedly having an affair.
In one of those text exchanges, Strzok says that there is "no way" Trump gets elected but that if he does there needs to be an "insurance policy" in place because "we cant take that risk."
Strzok was reassigned from the special counsel over the summer and relocated to the FBI's human resources department. Page, who referred to Trump as an "idiot" in the texts, also left the special counsel's team.
Trump hit out at Strzok, calling him a traitor.
"A man is tweeting to his lover that if [Hillary Clinton] loses, we'll essentially do the insurance policy. We'll go to phase two and we'll get this guy out of office," Trump said.
"That is a treasonous act. What he tweeted to his lover is a treasonous act," Trump added, confusing text messages and tweets.
Responding to a question, Trump reiterated that there was no collusion between him and the Russians and it is a Democratic party "hoax".
"I won a race that should never be won by a Republican because it is so stacked in the Democrats' favour...I won an election that should never be won, because the Electoral College is far harder to win than the popular vote. The popular vote, for me, would have been much easier," he said.
He also accused Democratic lawmakers for leaking information.
Trump claimed that no one has been as open as his administration.
He was responding to a question on Russian investigation.
"We gave them everything. We didn't go to court and say, "You can't have this document, you can't haveand what we gave them showedI never got a phone call from Russia. I didn't have a tweet. I didn't have an email. I didn't have a meeting," he said and alleged that the Democrats have made up a crime that does not exist.
Former FBI Director James Comey, whom he fired, said he has proven to be a liar and a leaker.
"Proven. He tries to act like a choir boy. What he did with Hillary Clinton is outrageous. He saved her life, because all of those chargesI call it Comey one, two, and three, all of those charges and Comey won, she was guilty of. She should have been taken out of the campaign and been on trial," he said.
"He didnt do that. He saved her life. But heres the way you look at Comey...Everybody hated Comey. The Democrats wanted him fired. Everybody wanted him out, said the US President.