U.S. President Donald Trump launched a fresh attack Sunday on the credibility of his own FBI, responding to revelations that an FBI agent was removed from special counsel Robert Mueller's team investigating Russian election meddling because of anti-Trump text messages.

Trump, two days after his former national security adviser Michael Flynn pleaded guilty to lying to the FBI, again denied that he directed then-FBI Director James Comey to stop investigating Flynn.

The Republican president offered a running Twitter commentary Sunday amid renewed focus on Mueller's probe and Flynn's decision to co-operate with the investigation as part of his plea agreement. Democrats said the developments suggested growing evidence of co-ordination between Trump's circle and Russian meddling in the 2016 presidential election.

"This president has been obsessed with this investigation, always saying there's nothing there, but each week, another shoe drops, where we see more evidence of continuing outreach from Russians and some response from the Trump campaign and Trump individuals," said Virginia Sen. Mark Warner, the top Democrat on the Senate Intelligence Committee.

In a series of tweets, Trump questioned the direction of the federal law enforcement agency and wrote that after Comey, whom Trump fired in May, the FBI's reputation is "in Tatters — worst in History!" He vowed to "bring it back to greatness." The president also retweeted a tweet that said new FBI Director Christopher Wray "needs to clean house."

FBI agent removed from Mueller's team

The president seized upon reports that a veteran FBI counterintelligence agent was removed from Mueller's team last summer after the discovery of an exchange of text messages that were viewed as potentially anti-Trump. The agent, Peter Strzok, had also worked on the investigation of Hillary Clinton's use of a private email server.

Peter Carr, a spokesperson for Mueller, said Mueller removed Strzok from the team "immediately upon learning of the allegations." He would not elaborate on the nature of the accusations. The person who discussed the matter with The Associated Press was not authorized to speak about it by name and spoke on condition of anonymity.

Trump tweeted Sunday: "Tainted (no, very dishonest?) FBI 'agent's role in Clinton probe under review.' Led Clinton Email probe." In a separate tweet, he wrote: "Report: 'ANTI-TRUMP FBI AGENT LED CLINTON EMAIL PROBE' Now it all starts to make sense!"

Strzok's removal almost certainly reflected a desire to insulate the investigators from any claims of political bias or favouritism. Trump and many of his supporters have at times sought to discredit the integrity of the investigation, in part by claiming a close relationship between Mueller and Comey and by pointing to political contributions to Democrats made by some lawyers on the team.

Mueller has been investigating whether Trump campaign associates co-ordinated with Russia to influence the outcome of the 2016 presidential election, and Strzok's background in counterintelligence would have been seen as particularly valuable for a secretive FBI probe examining foreign contacts.

Trump Russia Probe

Michael Flynn, who was forced out as national security adviser after just 24 days on the job, pleaded guilty on Friday to lying to the FBI about his contacts with Russian officials prior to the president's inauguration. (Carolyn Kaster/Associated Press)

Mueller's investigation has so far netted charges against four people, with the most recent criminal case brought Friday when Flynn pleaded guilty to lying to the FBI about his conversations with the Russian ambassador.

Trump lawyer says he wrote Flynn tweet

On Saturday, Trump tweeted that he "had to fire General Flynn because he lied to the Vice-President and the FBI. He has pled guilty to those lies. It is a shame because his actions during the transition were lawful. There was nothing to hide!"

The tweet suggested that Trump was aware when the White House dismissed Flynn on Feb. 13 that he had lied to the FBI, which had interviewed him weeks earlier. Comey has said Trump the following day brought up the Flynn investigation in private at the White House and told him he hoped he could "let this go."

Amid questions raised by the tweet, Trump associates tried to put distance Saturday evening between the president himself and the tweet. One person familiar with the situation said the tweet was actually crafted by John Dowd, one of the president's personal attorneys. Dowd declined to comment when reached by The Associated Press on Saturday night.

CNN reported on Sunday that Dowd confirmed to the network that he indeed was the author of the tweet.

Dowd told CNN that he drafted the tweet and that it was then posted by White House social media director Dan Scavino.

Trump Russia Probe Lawyer

Trump lawyer John Dowd told CNN that he drafted the tweet and that it was then posted by the White House social media director. (Richard Drew/Associated Press)

Rep. Adam Schiff, a Democrat from California and the top Democrat on the House Intelligence Committee, said given that Mueller could have charged Flynn with more crimes but instead limited it to just one offence, "Bob Mueller must have concluded that he was getting a lot of value in terms of Gen. Flynn's co-operation."

"I do believe he will incriminate others in the administration. Otherwise, there was no reason for Bob Mueller to give Mike Flynn this kind of deal," Schiff said, adding that, "whether that will ultimately lead to the president, I simply don't know."

Warner spoke on CNN's State Of The Union and Schiff spoke on ABC's This Week.​