Peter Strzok, the former FBI agent who exchanged critical private text messages about Donald Trump, has sued the FBI and US Justice Department, alleging he was fired due to “unrelenting pressure” from the president in violation of his Constitutional rights.

The lawsuit alleges he was unfairly punished for expressing his political opinions, and that the Justice Department violated his privacy when it shared hundreds of his text messages with reporters.

“The campaign to publicly vilify Special Agent Strzok contributed to the FBI’s ultimate decision to unlawfully terminate him,” the lawsuit says, “as well as to frequent incidents of public and online harassment and threats of violence to Strzok and his family that began when the texts were first disclosed to the media and continue to this day.”

The complaint, which names as defendants Attorney General William Barr and FBI Director Chris Wray, revisits a political drama that was seized on by conservative critics of special counsel Robert Mueller’s Russia investigation as proof that the bureau was biased against Mr Trump.

Multiple investigations are underway examining whether the FBI acted properly during the Russia investigation, and Mr Strzok remains a frequent target of Mr Trump’s scornful tweets.

Mueller investigation: The key figures Show all 12 1 /12 Mueller investigation: The key figures Mueller investigation: The key figures Robert Mueller is the special counsel overseeing the investigation into Russia's meddling in the 2016 election, and potential obstruction of justice by the president. Mr Mueller has a pristine reputation in Washington, where he was previously in charge of the FBI. Throughout his investigation, he and his team have been notoriously tight lipped about what they know and where their investigation has led. REUTERS Mueller investigation: The key figures Former FBI director James Comey was the catalyst that led to the appointment of special counsel Robert Mueller. Mr Comey was fired by the president after Mr Trump reportedly asked him to drop his own Russia investigation. Mr Trump has long maintained that the investigation is a "witch hunt". AFP/Getty Images Mueller investigation: The key figures Deputy attorney general Rod Rosenstein had authority over the special counsel investigation for much of the two years it has been active. Mr Rosenstein found himself with that responsibility after then-attorney general Jeff Sessions recused himself from that oversight. AP Mueller investigation: The key figures Attorney general Jeff Sessions's decision to recuse himself from oversight of the special counsel investigation may have cost him his job in the end. Mr Sessions resigned last year, after weathering a contentious relationship with Donald Trump who vocally criticised his attorney general for taking a step back. Mr Sessions recused himself from the oversight citing longstanding Justice Department rules to not be involved in investigations overseeing campaigns that officials were apart of. AP Mueller investigation: The key figures Attorney General William Barr is currently responsible for oversight of the special counsel investigation. Mr Barr's office will be the first to receive the Mueller report when it is finished. His office will then determine what portion or version of that report should be delivered to Congress, and also made public. EPA Mueller investigation: The key figures Michal Cohn is the president's former personal lawyer, who has been helping the special counsel investigation as a part of a plea deal over financial crimes, and campaign finance crimes, he has pleaded guilty to. Among those crimes, Cohen admitted to facilitating $130,000 in hush money payments to adult film actress Stormy Daniels during the 2016 campaign. Cohen has said he did so at the direction of Mr Trump. Cohen has also admitted that he maintained contacts with Russian officials about a potential Trump real estate project in Moscow for months longer than Mr Trump and others admitted. 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AP Mueller investigation: The key figures Former national security adviser Michael Flynn was one of the first casualties of the Russia scandal, and was forced out of his position in the White House weeks after Donald Trump took office. Flynn pleaded guilty in 2017 to "willfully" making fraudulent statements about contacts he had with Russian officials including former Russian ambassador to the US Sergey Kislyak. Flynn then lied to Vice President Mike Pence about that contact. REUTERS

Representatives of the FBI and Justice Department did not immediately respond to requests for comment.

The suit provides new details about the circumstances of Mr Strzok’s firing and amounts to the latest defence of his reputation, coming months after a fiery congressional hearing in which he insisted that his personal views never influenced his work.

Mr Strzok, a veteran counterintelligence agent who helped lead FBI investigations into Hillary Clinton’s use of a private email server and ties between the Trump campaign and Russia, was removed from Mr Mueller’s team after the texts with FBI lawyer Lisa Page came to light.

He was fired from the FBI last August.

Many of the texts, on FBI cell phones, were bitingly critical of Mr Trump during his 2016 run for office. They were found by the Justice Department’s inspector general during its investigation of the FBI’s Clinton email probe.

The watchdog office criticised both Mr Strzok and Ms Page, with whom he was having an affair, for their judgment in sending the messages but did not find that the Clinton email investigation was compromised by political bias.

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In the lawsuit, Mr Strzok's attorney Aitan Goelman says the FBI deputy director who fired him was responding to “unrelenting pressure from President Trump and his political allies in Congress and the media.”