Senate Intelligence Committee Chairman Richard Burr late Tuesday cast doubt on the veracity of a reported memo in which former FBI Director James Comey said President Trump requested he drop the FBI's investigation into former national security adviser Mike Flynn.

"I actually believe the director might have told us that there'd been a request like that and it was never mentioned by him," the North Carolina Republican told reporters. "So somebody's going to have to do more than have anonymous sources on this one for me to believe that there's something there."

Burr, who is leading a probe into Russian interference in the 2016 elections along side Virginia Democrat Mark Warner, said he'd not personally read the New York Times' report yet, but heard about it from fellow Sen. John McCain, R-Ariz.

According to the report, Comey authored a memo summarizing a conversation with the president, quoting Trump as asking him to end the investigation into Flynn, who served Trump during the campaign and in the earliest days of the administration. "I hope you can see your way clear to letting this go, to letting Flynn go," Trump is quoted as saying, according to the outlet. "He is a good guy. I hope you can let this go."

The gravity of the accusation inspired even Trump's more willing Republican critics to tread lightly. "I'm not going to opine about a memo," Sen. Lindsay Graham, R-S.C., said. "We're not going to try somebody on a piece of paper."

If the memo is correct, it's a serious charge against the president, but won't necessarily endear Comey to lawmakers on Capitol Hill. "If this happened, the FBI director should have done something about it or quit," Graham said. "If the president asked the FBI director to do something inappropriate, the FBI director should have said no and quit."

The Times report is based on "one of Mr. Comey's associates" who read parts of the memo to a reporter, but did not hand over the document.

Burr was not impressed by the sourcing. "I could write something and I could read it over the phone and tell them that it came from [Comey]," he told reporters. "I think the burden is on the New York Times, if they're reporting it and they've got somebody that's got the document, they need to get the document and get it released."

Burr noted that he met with Comey the day before Trump fired him. "The director of the FBI shared more information with Sen. Warner and myself than any director has ever shared," he said. "I think something as material as that probably would have been something he would have shared, had it happened," he said.

Al Weaver contributed to this report.