Once was during dinner; twice it happened during phone calls, according to the U.S. president.

Either way, Donald Trump's claim that he received three assurances from former FBI director James Comey that he's not under investigation as part of the Russia inquiry doesn't add up for veterans of the bureau.

In an interview with NBC News on Thursday, the president said Comey told him directly, "You are not under investigation," when Trump asked him about the probe examining his campaign's possible collusion with the Kremlin during the 2016 election.

"I had a dinner with him. He wanted to have dinner because he wanted to stay on ... as the FBI head," Trump said. "At that time, he told me, 'You are not under investigation.'"

But as the White House deals with credibility concerns following its shifting accounts of why Comey was fired on Tuesday, retired FBI agents are questioning why someone of Comey's standing and professional reputation would engage Trump in a conversation that so clearly presents a conflict of interest. Surely, they argue, Comey would have known better.

"My gut reaction? No way would Comey ever tell him that. It doesn't make any sense," said Myron Fuller, a former FBI supervisor who ran the Salt Lake City and Honolulu divisions. "Whether the president was or wasn't under investigation, it wouldn't be in protocol. It's not how you handle investigations."

'He's a showboat'

Trump's dismissal of Comey raised suspicions it was an attempt to thwart the ongoing investigation. It came days after Comey requested more resources to expand the probe.

"He's a showboat, he's a grandstander, the FBI has been in turmoil," Trump told NBC News on Thursday.

That statement, as well as Trump's assertion Comey had lost the trust of the FBI, went against sworn testimony on Thursday from the new acting FBI director, Andrew McCabe. Speaking before a Senate intelligence panel, McCabe confirmed his predecessor "enjoyed broad support within the FBI and still does until this day."

Standard FBI policy dictates that employees are not to confirm or deny details about active investigations. In his interview with NBC, Trump acknowledged he was the one who pressed Comey on the Trump-Russia probe during a phone call.

"I said, 'If it's possible, would you let me know, am I under investigation?'" he said.

'He knows the ropes'

Fuller, who served in the FBI for 30 years, says he doesn't doubt that Trump would have asked, but there are also long-standing White House policies in place that limit contact with the Justice Department on active investigations. The ethical guidance is not legally binding, though it's generally respected in order to shield investigations from political pressure.

"How and why would Comey go outside the professionalism of FBI standards ... when he knows the ground rules as a seasoned veteran of the Department of Justice?" Fuller asked.

Jim Pledger, a former FBI supervisor of the Washington Metro field office, wondered whether the alleged conversations didn't happen in quite the way Trump described. Comey would have likely sidestepped a direct question, telling Trump as he did during his testimony to Congress, that he can't comment on ongoing investigations.

"If you've seen him under sworn testimony, he just doesn't comment on things he's not supposed to. He knows the ropes."

During his testimony at the U.S. Senate intelligence committee on Thursday, acting FBI director Andrew McCabe said Comey was, and still is, highly respected. (Eric Thayer/Reuters)

What also shouldn't be overlooked is how unusual it would be not only for Comey to answer Trump's query about whether he was under investigation, but for the president to ask it in the first place, said David Gomez, a former assistant special agent at the FBI who ran the national security programs in Seattle.

Had Comey affirmed, for example, that Trump was indeed under investigation — what then?

"I find it incredulous that the president would even ask that question. If you're Mr. Comey, do you say, 'Yeah, you're the main target. In fact, we have wiretaps all over you'?" Gomez said. "I'm sure that kind of question would have made Mr. Comey very uncomfortable and he would have avoided being specific."

As for Trump's suggestion Comey requested a dinner meeting because he wanted to save his job at the FBI, Gomez doesn't buy that either.

FBI directorships come with 10-year terms and Comey had six years remaining and little reason to expect he was about to be terminated. Making an appeal over dinner to the president to keep his position strikes Gomez as sounding out of character for Comey.

"He's not the kind of guy who would ask the president over dinner to save his job, so I don't believe that for a second."

Gomez said based on his own talks with fellow FBI agents, Comey was shocked to learn he'd been terminated when he addressed the Los Angeles field office and saw the news flash on TV screens in the room.

Meanwhile, the White House line on why Comey was fired continues to shift, with the president telling NBC: "I was gonna fire, regardless of recommendation." In saying so, he contradicted Vice-President Mike Pence and White House press secretary Sean Spicer, who insisted Comey was booted on the recommendation of Trump's deputy attorney general, Rod Rosenstein.

Rosenstein, in turn, was reportedly so upset about getting pinned as the impetus for Comey's dismissal that he threatened to resign.

He will at least get a chance to clear his name likely early next week. Senate leaders Mitch McConnell and Chuck Schumer have invited Rosenstein to brief the full Senate on his version of the events leading up to Comey's firing.