White House Senior adviser Jared Kushner sits behind U.S. President Donald Trump during a cabinet meeting at the White House in Washington, November 1, 2017.

White House senior adviser Jared Kushner urged President Donald Trump to fire James Comey as FBI director, The Wall Street Journal reports, citing sources familiar with the matter.

The firing is reportedly being investigated by Special Counsel Robert Mueller as a possible attempt to obstruct justice.

Sources cited by the Journal gave differing reasons for Kushner's support of Comey's termination. One source said the move would be supported by disgruntled FBI agents who thought Comey mishandled the Hillary Clinton email investigation.

Comey's handling of that investigation was the reason the president originally cited for firing the FBI director in May. Trump also said he based his decision on a recommendation from the Justice Department.

Another source cited by the Journal said that Kushner, Trump's son-in-law, believed Comey's handling of the Clinton investigation proved that he was too "unpredictable."

Any obstruction of justice charge would be "exceedingly difficult" for the special counsel to prove, former Whitewater independent counsel Robert Ray told CNBC.

In an email, Ray said there are "a panoply of innocent reasons for wanting to remove an FBI Director that you concluded was 'unpredictable.'"

"The Government would have trouble proving beyond a reasonable doubt that one of those innocent reasons does not explain why the President decided to replace Director Comey, which the chief executive is absolutely entitled to do for any reason, or for no reason at all — so long as that reason does not constitute a knowing and willful attempt to obstruct justice," he said.

A 108-page report published in October by the Brookings Institution found that "the president likely obstructed justice."

The president's lawyers have repeatedly denied that charge.

Kushner's attorney, Abbe Lowell, told the Journal that Kushner supported the president's decision, while a White House attorney said that there was "no apparent evidence of Jared's involvement."

Attorneys for Kushner and the White House did not respond to requests for comment from CNBC.

The president offered some insights into his decision to fire the FBI director in the weeks that followed the announcement.

The day after the firing, the president told NBC News that "regardless of recommendation, I was going to fire Comey."

"When I decided to just do it, I said to myself, I said, you know: This Russia thing with Trump and Russia, it's a made-up story," he told NBC.

Later, the president told Russian officials during a meeting in the Oval Office that firing Comey had relieved "great pressure" on him, The New York Times reported.

The special counsel also obtained a letter that the president drafted, that was ultimately blocked by the White House counsel, which offered "an unvarnished view" of the president's thinking, according to the Times.

A lawyer for the president has called the letter "wholly exonerating."

The White House's top lawyer Don McGahn will be interviewed by the special counsel in the coming weeks and is expected to discuss the Comey firing, CNBC previously reported.

The special counsel's office declined to comment.