Who Is Peter Strzok, FBI Agent Accused of Treason by Trump?

A crest of the Federal Bureau of Investigation is seen 03 August 2007 inside the J. Edgar Hoover FBI Building in Washington, DC. Mandel Ngan/Getty Images

President Donald Trump accused an FBI agent of treason on Thursday for a series of private text messages he sent to his lover that gave a negative view of Trump during the 2016 election.

Trump told The Wall Street Journal that what agent Peter Strzok "tweeted to his lover is a treasonous act."

In December the Justice Department controversially released 375 text messages that Strzok and Department of Justice attorney Lisa Page exchanged from August 2015 to December 2016. Critics said the release violated their privacy.

The messages led to Strzok's removal from special counsel Robert Mueller's probe into ties between the Trump campaign and the Kremlin's efforts to interfere in the U.S. election last summer, The New York Times revealed.

The messages between the two sent during the 2016 described the possibility of a Trump administration as "terrifying" and that Hillary Clinton "just has to win."

Strzok was assigned in late July 2016 to supervise the FBI investigation of whether the Trump campaign was aiding Russian interference. The assignment came soon after he helmed an investigation into whether Clinton broke the law by mishandling classified information on a private server during her time as Secretary of State.

One message sent by Strzok that drew Trump's ire Thursday referred to the need for an "insurance policy" in case he was elected. Sources close to Strzok told The Journal last month that the text was a reference the FBI investigation of the Trump campaign and Russia and its need to move quickly to investigate whether the allegations the agency had uncovered were true.

Read more: Trump accuses FBI agents of treason over critical text messages

Few in the FBI were aware of the investigation, which eventually morphed into special counsel Mueller's probe in May 2017 following Trump's firing of FBI Director James Comey earlier that month.

Trump reportedly said soon after he fired Comey that he removed the FBI director to relieve pressure on him from the Russia investigation. Trump told NBC News during an interview that month that he removed Comey because of his handling of the Russia investigation. Trump said he would have done so "regardless" of a rationale outlined by deputy attorney general Rod Rosenstein in a letter at the time.

A view of the J. Edgar Hoover Building, the headquarters for the Federal Bureau of Investigation (FBI), on May 3, 2013 in Washington, DC. Brendan Smialowski/AFP/Getty Images

During the 1990s Strzok served an officer the U.S. Army before joining the FBI as an intelligence researcher.

By 2015 he was a senior analyst with experience supervising investigations that had involved Russian and Chinese espionage. Strzo was enlisted that year to oversee the investigation code-named "Midyear" which was investigating Clinton's private server.

By the spring of 2016, Strzok reported to Comey that Clinton had been careless with classified information but that the team hadn't found evidence of her intent to mishandle the information which was key to bringing charges.

After Mueller was made aware of Strzok's text messages in the summer of 2017, he was reassigned back to the FBI in a post in the human resources division.