WASHINGTON (Reuters) – U.S. President Donald Trump is being investigated by special counsel Robert Mueller for possible obstruction of justice, the Washington Post reported on Wednesday, citing unidentified officials.

Mueller is investigating alleged Russian interference in the 2016 U.S. presidential election and possible collusion with the Trump campaign. Former FBI Director James Comey told Congress last week he believes he was fired by Trump to undermine the agency’s Russia probe.

The Washington Post, citing five people briefed on the requests who spoke on condition of anonymity, said Dan Coats, the director of national intelligence, Mike Rogers, the head of the National Security Agency, and Richard Ledgett, the former deputy director at the NSA, had agreed to be interviewed by Mueller’s investigators as early as this week.

The obstruction of justice investigation into Trump began days after Comey was fired on May 9, according to people familiar with the matter, the Washington Post said.

Trump’s legal team quickly denounced the report on Wednesday.

“The FBI leak of information regarding the President is outrageous, inexcusable and illegal,” a spokesman for Trump’s legal team, Mark Corallo, said.

A spokesman for Mueller’s team declined to comment.

Several legal experts told Reuters that Comey’s testimony last week that Trump expected loyalty and told Comey he hoped he could drop an investigation of a former top aide could bolster obstruction of justice allegations against Trump.

Comey would not say in his testimony last week whether he thought the president sought to obstruct justice, but added it would be up to special counsel Mueller “to sort that out.”

After Comey’s testimony, Trump said he had been vindicated because his former FBI director confirmed telling Trump on three occasions that he was not under investigation.

While a sitting president is unlikely to face criminal prosecution, obstruction of justice could form the basis for impeachment. Any such step would face a steep hurdle as it would require approval by the U.S. House of Representatives, which is controlled by Trump’s fellow Republicans.

(Additional reporting by Steve Holland Nathan Layne; Reporting by Eric Beech; Editing by Howard Goller)