There were menacing phone calls and messages from strangers, and anxious peeks out window shades before his family would leave the house.

FBI security experts advised him of best practices - walk around your car before entering, watch for unfamiliar vehicles in your neighbourhood - more commonly associated with mob targets looking to elude detection.

"Being subjected to outrageous attacks up to and including by the president himself, which are full of lies and mischaracterisations and just crude and cruel, is horrible," Strzok told The Associated Press in an interview.

"There's no way around it."

A new book by Strzok traces his arc from veteran counterintelligence agent to the man who came to embody Trump's public scorn of FBI and his characterisation of its Russia investigation as a "witch hunt."

The texts cost Strzok his job and drew vitriol from Trump.

But even among Trump critics, Strzok isn't a hero. His anti-Trump texts on a government phone to an FBI lawyer gave Trump and his supporters a major opening to undercut the bureau's credibility right as it was conducting one of the most consequential investigations in its history.

Trump's attacks have continued even as two inspector general reports found no evidence Strzok's work in the investigations were tainted by political bias and multiple probes have affirmed the Russia probe's validity.

Strzok expresses measured regret for the texts in "Compromised: Counterintelligence and the Threat of Donald J. Trump," due out Tuesday.

"I deeply regret casually commenting about the things I observed in the headlines and behind the scenes, and I regret how effectively my words were weaponised to harm the Bureau and buttress absurd conspiracy theories about our vital work," Strzok writes.

Before becoming a virtual household name, Strzok spent two decades at the FBI toiling in relative anonymity on sensational spy cases.

He helped uncover Russian sleeper agents inside the US, worked the Edward Snowden case and led the investigation into whether Hillary Clinton mishandled classified information. (She did, he writes, but not in a way meriting prosecution).

After the Clinton case concluded in July 2016, Strzok opened an investigation into whether the campaign of her Republican opponent was coordinating with Russia, conceiving the "Crossfire Hurricane" codename he says proved prescient.

Strzok said he intended for his book to lend insight into the Clinton probe, Russian election interference and, "first and foremost, the counterintelligence threat that I see in Donald Trump."

"To do that," he said in the interview, "I wanted to show the reader what happened but also why they should believe me."

Australian Associated Press