The FBI and Congress are examining a campaign event last spring during which Donald Trump, Jeff Sessions and Jared Kushner were in a small gathering with Russian ambassador to the U.S. Sergey Kislyak and other diplomats at Washington’s Mayflower Hotel, multiple U.S. officials told NBC News.

Five current and former U.S. officials said they are aware of classified intelligence suggesting there was some sort of private encounter between Trump and his aides and the Russian envoy, despite a heated denial from Sessions, who has already come under fire for failing to disclose two separate contacts with Kislyak. Kushner also denied through a spokesman that he met privately with Kislyak that day.

The officials acknowledged to NBC News that the evidence does not amount to proof, and they have declined to provide details about it.

"The Department of Justice appointed special counsel to assume responsibility for this matter," Department of Justice spokeswoman Sarah Isgur Flores said in a statement. "We will allow him to do his job. It is unfortunate that anonymous sources whose credibility will never face public scrutiny are continuously trying to hinder that process by peddling false stories to the mainstream media. The facts haven't changed; the then-Senator did not have any private or side conversations with any Russian officials at the Mayflower Hotel."

CNN reported Wednesday about the investigative interest in the Mayflower event, which took place on April 27, 2016. NBC News has been discussing the matter with knowledgeable sources for weeks, seeking more clarity about why Congressional investigators believe there may have been a private meeting.

Sergey Kislyak, Russia's ambassador to the U.S. speaks with reporters following his address on the Syrian situation, on Sept. 6, 2013, at the Center for the National Interest in Washington. Cliff Owen / AP, file

A U.S. official with knowledge of the matter told NBC News that the FBI also is scrutinizing the Mayflower event, which was sponsored by a pro-Russian think tank. The official said the FBI is interested in who was at the event and what was said, in the context of the counter-intelligence investigation into Russian election meddling. That official said there was no indication the bureau is zeroing in on Sessions.

Sen. Al Franken, D.-Minnesota, who originally questioned Sessions about his Russian contacts during a confirmation hearing for Sessions' appointment as attorney general, discussed the matter Wednesday night on MSNBC’s The Last Word with Lawrence O’Donnell

"It had been characterized one way, but we had some reason to believe that wasn’t the case," Franken said about the event. "It had been described in a way that he could say, 'I don’t remember that.'"

It has long been known that Trump briefly met Ambassador Kislyak that day at a VIP reception shortly before he gave a foreign policy address at the hotel. But witnesses said it wasn’t a private meeting, and White House officials dismissed it as inconsequential.

"Mr. Trump warmly greeted Mr. Kislyak and three other foreign ambassadors who came to the reception," the Wall Street Journal reported in May 2016.

Kushner and Sessions were also in the room, contemporaneous news reports say. Sessions’ aides have insisted he did not speak to Kislyak.

Congress is investigating the credibility of intelligence seeming to contradict that account, current and former U.S. officials say. And Franken, in a March letter to the FBI with Judiciary Committee Democrat Patrick Leahy, asked the bureau to investigate any contacts between Sessions and Russian officials, and to brief him on the results. He has not yet received an answer, an aide said.

Click Here To Read The Letter

The FBI investigation into Russian election interference is now supervised by Special Counsel Robert Mueller, and Sessions is walled off from it, having recused himself.

In March, the Center for National Interest, the right-leaning, Russia-linked group that hosted the event, said that the receiving line "moved quickly and any conversations with Mr. Trump in that setting were inherently brief and could not be private. Our recollection is that the interaction between Mr. Trump and Ambassador Kislyak was limited to the polite exchange of pleasantries appropriate on such occasions."

Republican presidential candidate Donald Trump gives a thumbs up after a foreign policy speech at the Mayflower Hotel in Washington, Wednesday, April 27, 2016. Trump's highly anticipated foreign policy speech Wednesday will test whether the Republican presidential front-runner, known for his raucous rallies and eyebrow-raising statements, can present a more presidential persona as he works to unite the GOP establishment behind him. (AP Photo/Evan Vucci) Evan Vucci / AP

Sessions recused himself from the Russia investigation after it emerged that he had met twice with Kislyak after telling senators under oath during his confirmation hearing that he had not met with Russian officials about the Trump campaign.

"In retrospect," Sessions told reporters, "I should have slowed down and said, ‘But I did meet one Russian official a couple of times, and that would be the ambassador.’"

Lawmakers involved in the Russia investigation would not discuss the April meeting.

"I can’t comment on any of that," Sen. Mark Warner, the ranking Democrat on the Senate Intelligence committee, told NBC News.

"Can’t talk about it," said Sen. Ron Wyden, D-Oregon, a member of the committee.

In a little noticed portion of a March congressional hearing on the Russia investigation, Rep. Mike Quigley, a Chicago Democrat on the House Intelligence Committee, accused Sessions of having committed perjury about an alleged undisclosed third meeting in April.

He noted that Sessions had failed to disclose meetings with Kislyak in July and September, during a time the Russians were "hacking and dumping" stolen emails in the election campaign.

He added, "Unfortunately, what we're reading now is that there was a third meeting as early as April of last year in Washington, D.C., a meeting at which Candidate Trump was present and the Russian ambassador was present. At some point in time, this goes well beyond an innocent, under the best of circumstances, ‘Oh I forgot’ sort of thing, or `That doesn't count.’ When you correct your testimony in front of the United States Senate, you're still under oath and you're swearing to the American people that what you're saying is true. Well, the third time is well beyond that and is quite simply, perjury."

Quigley said he could not discuss the basis of his remarks about the April event, other than to say he wasn’t relying solely on news reports.

Any confirmation of a private meeting with Kislyak in April would raise a host of questions, most particularly for Sessions.

April 2016 is when officials at the Democratic National Committee first noticed suspicious activity on their network — activity they would later learn was part of a Russian hack.

At Sessions' confirmation hearing in January, Sen. Franken asked him, "If there is any evidence that anyone affiliated with the Trump campaign communicated with the Russian government in the course of this campaign, what will you do?"

Sessions replied: "Senator Franken, I’m not aware of any of those activities. I have been called a surrogate at a time or two in that campaign and I did not have communications with the Russians, and I’m unable to comment on it."

In March, the Washington Post reported that Sessions had met twice with Kislyak — once in the senator’s office in September, and once in July at a Heritage Foundation event.

On March 2, at a news conference announcing his recusal from the Russia investigation, Sessions said: "I never had meetings with Russian operatives or Russian intermediaries about the Trump campaign. And the idea that I was part of a quote, ‘continuing exchange of information’ during the campaign between Trump surrogates and intermediaries for the Russian government is totally false."

He acknowledged, however, that the two meetings reported by the Post had occurred. He said he didn’t recall much of what was discussed. He said he took the meetings as a senator, not a Trump adviser.

A reporter asked if he recalled meeting with Kislyak any other times.

"I don't recall having met him," Sessions answered. "It's possible — I'm on the Armed Services Committee and things happen, but I don't recall having met him before those two meetings."