WASHINGTON (NYTIMES) - A former senior FBI agent at the centre of the investigations into Mrs Hillary Clinton's email server and the Trump campaign's ties to Russia defends the handling of the inquiries and declares President Donald Trump a national security threat in a new memoir, while admitting that the bureau made mistakes that upended the 2016 presidential election.

The former agent, Peter Strzok, who was removed from the special counsel's team and later fired over disparaging texts he sent about Mr Trump, has mostly kept silent as the president and his supporters have vilified him.

But Mr Strzok's new book, "Compromised", a copy of which was obtained by The New York Times before its publication on Tuesday (Sept 8), provides a detailed account of navigating the two politically toxic investigations and a forceful apologia of the bureau's acts.

Mr Strzok also reveals details about the FBI's internal debate over investigating the president himself, writing that the question arose early in the Trump presidency and suggesting that agents were eyeing others around Mr Trump. Mr Strzok was himself at first opposed to investigating the president.

But in a scathing appraisal, Mr Strzok concludes that Mr Trump is hopelessly corrupt and a national security threat.

The investigations that Mr Strzok oversaw showed the president's "willingness to accept political assistance from an opponent like Russia - and, it follows, his willingness to subvert everything America stands for".

"That's not patriotic," Mr Strzok writes. "It's the opposite."

AN INSIDER LOOK

Mr Strzok's insider look serves as a counter to the efforts by Mr Trump and his allies to discredit the Russia investigation.

Attorney-General William Barr has appointed a veteran prosecutor to review the conduct of the FBI, Mr Strzok and others for possible misconduct and bias.

The Justice Department inspector-general Michael Horowitz found the bureau had sufficient reason to open the inquiry and found no evidence of political bias. He said in a report that he found no evidence that Mr Strzok's political views affected the FBI's work but that he was "deeply troubled" by the texts.

Mr Trump and his supporters seized on the texts when they were first disclosed in late 2017 as evidence of a plot to destroy his campaign and presidency.

"The reporting about my texts hadn't only whipped Trump into a frenzy," Mr Strzok writes. "It had also sent Republicans in Congress into a righteous peeve, giving them fodder for right-wing indignation that would eventually ferment into the deep-state fairy tale that would consume conservative media."

On Twitter, Mr Trump celebrated the firing of Mr Strzok, a former Army intelligence officer and veteran counterintelligence investigator who worked on some of the FBI's most delicate national security matters during his 22-year career before playing a central role in both the Clinton email and Trump-Russia investigations.

Mr Strzok has sued the Justice Department and FBI, alleging that his dismissal was politically motivated.

THE CONSPIRACY PROBE

In his book, Mr Strzok repeatedly rejects accusations that he was part of an effort at the FBI to hurt Mr Trump. He lays out the reasoning for opening the investigation, known as Crossfire Hurricane, into whether any Trump campaign associates had conspired with Russia's interference operations in the 2016 election.

The FBI was "investigating a credible allegation of foreign intelligence activity to see where it led," Mr Strzok writes. "It started with Russia, and it was always about Russia."

He also points out that the FBI had kept the investigation as quiet as possible to keep from harming Mr Trump's candidacy, limiting the number of people inside the bureau who were aware of it to try to ensure its existence did not leak to the news media.

When FBI officials later considered opening a counterintelligence investigation on the president, they faced a sobering reality.

"We needed to ask a question that had never before arisen in the entire 240-year history of our republic: whether the president of the United States himself might be acting as an agent of a foreign adversary," Mr Strzok writes.

He says he was opposed for both "practical and philosophical arguments".

Eventually, the FBI did begin investigating the president after he fired Mr James Comey as its director in May 2017, a step that also prompted the appointment of a special counsel.

'A COMPLICATED PORTRAIT OF FOREIGN INTERFERENCE'

Mr Strzok recounts first briefing the special counsel, Mr Robert Mueller, a former FBI director, and outlining the "dizzyingly complicated portrait of foreign interference".

"And on top of it all, at the pinnacle of this heap of perfidy and treachery, sat a president who had lied to the public, cozied up to Russia, and, once he became aware of them, attempted to block our investigation at every turn," Mr Strzok writes.

Mr Strzok helped select the FBI personnel who worked for Mr Mueller. He says that he wanted to make sure the counterintelligence aspects of the work would be "sufficiently addressed", and that it was not enough to simply send back what they found to the FBI.

When Mr Mueller removed Mr Strzok from the team after learning about his texts attacking the president, "we were still looking for the right way to investigate those counterintelligence concerns," Mr Strzok writes.

But investigators' pursuit of the matter withered after Mr Strzok left, in part because the deputy attorney general at the time, Mr Rod Rosenstein, had instructed Mr Mueller earlier to focus on possible crimes.

Counterintelligence investigations look for possible threats to national security.

The FBI was unprepared for the Russian blitz on the election, despite warning signs, Mr Strzok writes. He says the bureau should have seen the attack coming and the effect it would have.

He adds that the US government was also not "collaborating as effectively as we should have been".

FBI HURT HILLARY CLINTON'S CAMPAIGN

Mr Strzok also devotes considerable time in the book to the FBI's investigation into Mrs Clinton's use of a private email server, known inside the bureau as Midyear Exam. He concedes that Mr Comey erred by holding a news conference in July 2016 to say that the FBI would not recommend Mrs Clinton be charged with mishandling classified information but that her conduct was "extremely careless".

Critics of Mr Comey called the remarks an ad hominem attack that damaged her campaign. Mr Comey's speech, along with a pair of letters to Congress just before the election revealing that the investigation was briefly reopened and then closed, most likely cost her the election, Mr Strzok says.

"And as much as it pains me to admit, the Russians weren't the only ones who pushed the needle toward Trump," he writes. "The bureau did, too."

Mr Strzok says he was proud of the investigation, which he says was handled professionally, but he laments that the resources devoted to it could have been used to fight greater threats like China or Russia.

He says there was no comparison between the Russia investigation and the email inquiry, which the Republicans had used as an anvil to damage Mrs Clinton's election chances.

"Midyear was a mishandling case with little if any impact on national security," Mr Strzok writes. "In contrast, Crossfire was looking into whether anyone in the Trump campaign was conspiring with the Russians - even up to the unlikely worst-case scenario that Trump was a Manchurian candidate."

Mr Strzok says the attacks on him and his family have been painful and that he received threats.

He writes that he regrets sending the texts that ended his career and the damage inflicted on the FBI because of them.

When he was fired, Mr Strzok writes, "After a quarter of a century in pursuit of the nation's enemies, I had been deemed an enemy myself."