Congressional investigators have spent close to six months now, digging into possible ties between the campaign of President Donald Trump and the Russian government, and a handful of key figures have captured their attention.

With the blockbuster testimony of the former FBI director behind them, here are the top five subjects under scrutiny from the Hill:

1. Michael Flynn

Retired Lt. Gen. Michael Flynn, former national security advisor, arrives to a swearing in ceremony of White House senior staff in the East Room of the White House on January 22, 2017 in Washington. (Photo by Andrew Harrer-Pool/Getty Images)

Former national security adviser Michael Flynn is at the top of just about every investigators' list of possible witnesses. The House and Senate Russia probes have both subpoenaed him and his businesses. The House oversight committee has been on Flynn since before Trump even took office -- and successfully obtained his security clearance application, exposing that he may have broken the law by leaving off payments from Russia's RT network.

Flynn's legal team first floated testifying if granted immunity from prosecution, although no one has yet taken them up on that offer. His legal team initially rebutted efforts to obtain his documents, arguing that the Fifth Amendment protected him from both testifying and providing documents that could prove incriminating.

But eventually they conceded, agreeing to a document production schedule with the Senate and providing documents to the House Intelligence Committee this week as well.

Comey testified that Trump asked him to drop his criminal investigation of Flynn. in his hearing, the former FBI director later added that Flynn is being investigated for lying to federal investigators.

2. Jeff Sessions

Attorney General Jeff Sessions speaks during the Daily Briefing at the White House in Washington, DC, March 27. (Photo credit should read JIM WATSON/AFP/Getty Images)

Comey whet their appetites Thursday, when he told the Senate intelligence committee that Justice Department officials long expected Sessions to recuse himself from overseeing the Russia probe for multiple reasons -- implying it was more than just because of Sessions' role in the Trump campaign, as he said was his reason for stepping aside.

Then, in a closed session, Comey told them the FBI believes Sessions had a third, undisclosed meeting with -- something CNN reported last week that Hill investigators have been intensely probing.

Despite being one of their former colleagues, Sessions has sparked the ire of Democratic lawmakers in particular, who say he may have perjured himself when he testified in his confirmation hearing that he never had any communications with Russians during the campaign.

The revelation that Sessions met with Kislyak twice during the campaign -- and possibly a third time -- led Democratic Sens. Al Franken and Patrick Leahy to request that the FBI look into whether Sessions lied about his contacts.

"We are concerned about Attorney General Sessions' lack of candor to the committee and his failure thus far to accept responsibility for testimony that could be construed as perjury," Franken and Leahy wrote to Comey in their first request.

3. Jared Kushner

Jared Kushner, son-in-law and senior aide to US President Donald Trump, looks on before a meeting of the House and Senate leadership with Trump in the Roosevelt Room at the White House on Tuesday. (NICHOLAS KAMM/AFP/Getty Images)

Interest in Trump's powerful son-in-law has ramped up recently, following revelations that he attempted to set up a backchannel line of communications directly to Moscow through Kislyak.

Senate staff expect to interview Kushner soon , sources said Thursday.

Federal investigators have been looking at Kushner's role overseeing the Trump campaign's data operation and have also expressed interest in a December meeting with Sergey Gorkov, the head of Russia's state-owned Vnesheconombank (VEB) and a former Russian spy appointed to the bank by Russian President Vladimir Putin.

Sen. Angus King, a Maine independent who caucuses with the Democrats, asked Comey Thursday about VEB, but Comey said he could not answer that question in public.

For Kushner and Sessions, the intense interest also derives from them still working closely with Trump. The White House has largely distanced itself from other people under scrutiny.

4. Carter Page/Roger Stone/Paul Manafort

A trio of Trump "formers" have long been on the radar of Hill investigators, but interest has lagged somewhat as more prominent figures like Kushner and Sessions have risen.

Manafort has provided documents to the Senate intelligence committee and hired a crisis communications expert in addition to his legal team. Stone, who is being probed for his contacts with Guccifer 2.0 , has also turned over documents to Senate investigators.

5. Susan Rice

Susan Rice, Obama's former national security adviser, has nothing directly to do with any meddling by Russian operatives in the US election. But, because she has been targeted by Trump as part of the White House's counter-narrative, she has been swept up as a key figure, albeit not exactly by the Hill's Russia investigators.

Nunes, who still holds considerable power to direct investigations and issue subpoenas on his own, sent subpoenas to the CIA, FBI and NSA seeking details on who Rice, former UN Ambassador Samantha Powers and former CIA Director John Brennan sought to have " unmasked " -- or identify redacted names in collected intelligence -- in reports.

Nunes, in a rare public statement about the Russia probe and his work on the House intelligence committee tweeted at the time, "Seeing a lot of fake news from media elites and others who have no interest in violations of Americans' civil liberties via unmaskings."