Sen. Lindsey Graham Lindsey Olin GrahamSteele Dossier sub-source was subject of FBI counterintelligence probe Hillicon Valley: Subpoenas for Facebook, Google and Twitter on the cards | Wray rebuffs mail-in voting conspiracies | Reps. raise mass surveillance concerns Key Democrat opposes GOP Section 230 subpoena for Facebook, Twitter, Google MORE (R-S.C.) said Thursday that he believes former special counsel Robert Mueller Robert (Bob) MuellerCNN's Toobin warns McCabe is in 'perilous condition' with emboldened Trump CNN anchor rips Trump over Stone while evoking Clinton-Lynch tarmac meeting The Hill's 12:30 Report: New Hampshire fallout MORE was just a "figurehead" at the top of the investigation into the Trump campaign and pledged to investigate the probe's origins.

Graham told reporters on Capitol Hill that the GOP-controlled Senate would continue efforts alongside Attorney General William Barr Bill BarrHillicon Valley: Subpoenas for Facebook, Google and Twitter on the cards | Wray rebuffs mail-in voting conspiracies | Reps. raise mass surveillance concerns Bipartisan representatives demand answers on expired surveillance programs YouTube to battle mail-in voting misinformation with info panel on videos MORE to investigate whether the probe into the Trump campaign was begun improperly.

ADVERTISEMENT

"Yeah, I talked to him last night, we talked about ... I told him I'll try to find out how all this mess started, and went so long," Graham said, apparently referring to President Trump Donald John TrumpSteele Dossier sub-source was subject of FBI counterintelligence probe Pelosi slams Trump executive order on pre-existing conditions: It 'isn't worth the paper it's signed on' Trump 'no longer angry' at Romney because of Supreme Court stance MORE.

"It's clear to me that Bob Mueller was pretty much a figurehead of the investigation," Graham added. "We'll dig in and find what happened."

Barr announced earlier this year that the Justice Department would begin a review of the early stages of the investigation into Russian interference into the presidential election.

The FBI's probe into Russian meddling began in 2016. The special counsel's investigation began when then-Deputy Attorney General Rod Rosenstein Rod RosensteinDOJ kept investigators from completing probe of Trump ties to Russia: report Five takeaways from final Senate Intel Russia report FBI officials hid copies of Russia probe documents fearing Trump interference: book MORE appointed Mueller after Trump fired James Comey James Brien ComeySteele Dossier sub-source was subject of FBI counterintelligence probe Judge will not dismiss McCabe's case against DOJ Democrats fear Russia interference could spoil bid to retake Senate MORE as FBI director.

Republicans have argued for months that the Russia investigation, which ended earlier this year without any criminal charges targeting further members of the Trump administration, was improperly launched during the Obama administration as a means of spying on the Trump campaign.

The Mueller probe resulted in criminal charges and convictions for multiple former members of the president's inner circle before its shuttering this spring.

— This story was updated at 1:16 p.m.