(CNN) Members of the Senate intelligence committee are expected to have a briefing Tuesday at the CIA, sources familiar with the matter told CNN on Tuesday, as part of the panel's investigation into Russian meddling in the US election last year.

Members of the committee -- including Chairman Richard Burr of North Carolina and ranking Democrat Mark Warner of Virginia -- were spotted boarding a chartered bus to CIA headquarters in Langley, Virginia. They declined to answer from the media on their way to the bus.

"Slow news day?" Burr, a Republican, responded when reporters asked about the trip.

The Senate intelligence committee is one of four congressional investigations currently underway into Russian meddling in the US election.

Some Democrats have criticized the pace of the Senate panel's investigation, prompting the committee to announce last week that it'll hire two more staffers for its investigation.