Quits ahead of agency review by FBI deputy director Andrew McCabe

Mr. McCabe was reportedly due to retire on 18 March.

His exit from the top law enforcement agency comes a week after a report that Mr. Trump wanted him out.

An internal communication authored by Mr. Wray shows Mr. McCabe's early departure was the result of a forthcoming FBI inspector general report that concluded the agency must perform at the highest standards, according to CBS News.

Andrew McCabe was pressured to quit by FBI Director Christopher Wray in advance of an inspector general report examining the agency, reports CBS News.

Why is this significant?

Mr. McCabe briefly became acting FBI director last May after Mr. Trump fired its previous chief, James Comey.

Mr. Comey had been overseeing the bureau's investigation into alleged Russian interference in the 2016 presidential election.

Mr. Trump eventually nominated Christopher Wray as the new FBI director, and he was confirmed by the Senate in August.

The Federal Bureau of Investigation's deputy director, whom US President Donald Trump repeatedly accused of political bias, has resigned.

Did McCabe have a conflict of interest?

Recently released text messages indicate FBI officials felt Mr. McCabe should have recused himself earlier from the Clinton email investigation owing to his wife's political aspirations.

In a 28 October 2016 exchange, FBI agent Peter Strzok wrote to colleague Lisa Page that the bureau's chief of staff "very clearly 100% believes that Andy should be recused because of the 'perception'".

Mr. McCabe eventually did recuse himself from the Clinton probe a week before the November 2016 election.

Republican-led congressional committees have launched inquiries into whether the FBI botched its Clinton investigation and showed favoritism towards her.

Democrats have said those investigations are attempts to distract from US Department of Justice Special Counsel Robert Mueller's investigation into whether Mr. Trump's 2016 presidential campaign colluded with Russia.

Andrew McCabe had become a lightning rod for Republican criticism of the FBI's handling of the investigation into possible Trump campaign ties to Russia. Now he's heading for - or, perhaps, being pushed toward - the exits a bit earlier than he planned.

Donald Trump had already been explicit in his criticism of the bureau's second-in-command during the tumultuous 2016 presidential election season. In a late December tweet, the president essentially counted the days until Mr. McCabe's planned springtime retirement.

FBI Director Christopher Wray reportedly threatened to resign rather than sack his deputy director. Something changed.

Perhaps the political pressure from Republicans became too much. Or maybe, as Democrats have warned, this is the beginning of a political-tinged purge of the nation's top law-enforcement agency.

If there's something more concerning about Mr. McCabe's tenure, beyond a Democratic-affiliated wife or his name surfacing in text messages in 2016 between two FBI employees who had been critical of Mr. Trump, it's only a matter of time before it comes to light.

What's clear at this point, however, is that the Russian investigation is increasingly becoming mired in partisan trench warfare - and it could directly affect how Robert Mueller's special investigation proceeds.