Story highlights The Senate intelligence committee is reviewing documents related to Russia meddling in the US election

Some have been frustrated at the pace of the so-far three-month investigation

(CNN) The Senate Intelligence Committee is hiring two new staffers for its investigation into Russian interference in the US election, the top Democrat on the Senate Russia investigation told CNN on Monday.

The additional staffers -- including one Republican and one Democrat, versed in the National Security Agency collection tactics -- come as some sources on the committee have grumbled behind the scenes about the pace of the investigation.

Sen. Mark Warner, a Virginia Democrat, said investigators obtained a large batch of documents they requested just before Congress went on break and have completed 27 interviews as part of their investigation.

"I think things need to pick up ... now that we've gotten ... through the first cut of all the information," Warner told CNN in an interview. "Our approach, which is going to continue to be methodical, though, because when you bring in some of the names that have been bandied about in the press, you don't know how many bites at the apple you're going to get. You want to make sure you ask the right questions."

A Senate intelligence committee source said Monday that Warner was frustrated with the pace of the investigation -- even as it has been viewed publicly as the more collegial investigation, when compared to the House Russia investigation. The Senate investigation has moved slowly, and without the drama of the House version, but Warner said he expects things to pick up.