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.