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.
"All those questions are now going to be coming to a head, because we've now gotten through the first round of document review," Warner said.
Senate investigators, the source said, are a long way from bringing in high-profile targets like former Trump campaign chairman Paul Manafort
, former Trump national security adviser Michael Flynn
, former Trump campaign adviser Roger Stone
and former Trump campaign adviser on national security Carter Page
Other Democrats on the committee have been urging lawmakers to move more aggressively.
"I've made clear to leadership I have concerns about the pace," Sen. Ron Wyden, a Democrat on the Intelligence Committee, told CNN. "I believe you have to speed it up right now. Americans are getting most info from leaks and false tweets."
Wyden said that he would like to see investigators begin using subpoenas and hosting more public hearings to extract information.
Wyden also declined to say whether he's confident in Senate intelligence committee chairman Richard Burr's leadership atop the panel. He said he's counting on Warner to help pick up the pace.
"I don't think it's going as quickly," said California Sen. Dianne Feinstein, who declined to elaborate.
"I am concerned it's not moving along fast enough," said New Mexico Sen. Martin Heinrich. "I think it is making progress. But I think there needs to be more resources, more staffing."
The frustrations appear far from boiling over into the dysfunction that almost derailed the House Intelligence Committee's investigation a month ago.
"I wish there were a way that it could go even more quickly but I think it's important that we be thorough," said Sen. Susan Collins, a Maine Republican on the Senate intelligence committee. "And I was very happy we had the open hearing to hear from Russia experts, which put in context the efforts of Russia over the ages. The two leaders of the committee work extremely well together and we get a briefing all the time from them every week on where we are."
Sen. Angus King, a Senate intelligence member and Maine Independent who caucuses with the Democrats, downplayed any partisan tensions.
"I don't know of any partisan delays or any of that," King said. "The staff is working very diligently at it."
And Oklahoma Republican Sen. James Lankford tweeted, "Don't confuse silence for lack of progress."
Senate investigators have gained access to the intelligence typically reserved for the small "Gang of Eight" -- an exclusive group of House and Senate leaders with top-level intelligence clearance, which include leadership as well as the chairmen and ranking members of each chamber's intelligence committee. And staff have been talking with the analysts.
In the Capitol, the House Russia investigation has drawn the most attention by far -- beginning with FBI Director James Comey's stunning revelation last month that the FBI has been investigating Trump aides since last July
, through to House intelligence Chairman Devin Nunes' surprise recusal from the investigation
as he became the subject of a House ethics investigation stemming from his secret trip to the White House in March.
The consternation comes as lawmakers return from their two-week break, which saw much of the heated debate about the investigations die down, capped by Nunes' recusal just hours before House lawmakers left town.
The gears have been turning slowly behind the scenes in the Capitol, but mounting anger in public outside Washington has been clear in town hall confrontations and polling.
An NBC/Wall Street Journal poll released Monday
found that 73% of respondents want an independent commission instead of Congress to investigate Russia's interference. Only 16% of respondents said they preferred Congress.
The Senate Intelligence Committee has hosted a pair of public hearings in its investigation so far. The House invited former acting Attorney General Sally Yates, former Director of National Intelligence James Clapper and former CIA Director John Brennan for its second public hearing May 2, following weeks of partisan wrangling over the hearing in private.