The House and Senate are vowing a thorough investigation into alleged Russian interference in the 2016 election, with hearings on the House side expected to begin Tuesday.

Intelligence agencies have concluded that Russian hackers had meddled in last year’s election on President Donald Trump’s behalf, according to reports, though the extent still isn’t clear. FBI Director James Comey is among those expected to testify before congressional intelligence committees, as well as allies and key officials of the Trump administration.

Here are some of the key figures whose actions and words could play roles in the hearings.

Carter Page

A former foreign policy adviser to the Trump campaign, Page traveled to Moscow last summer, giving a Russian-friendly speech and meeting with a confidant of Russian President Vladimir Putin. After Page left the campaign, intelligence officials obtained a warrant to monitor his communications, reportedly on the suspicion that he was a Russian agent. They reportedly cited communications intercepted in 2013 concerning a Russian officer trying to recruit Page, who has vigorously denied any wrongdoing. Page worked in Moscow for three years (2004-7) as a Merrill Lynch executive.

Trump personally announced Page as a member of his campaign’s foreign policy team in March 2016 but has since distanced himself.

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Paul Manafort

Trump chose the longtime Republican operative to take over his campaign in March 2016 as an effort to broaden and professionalize an operation that was gaining steam. But Manafort resigned in August after reports that he was part of a covert Washington lobbying operation on behalf of pro-Russia factions in Ukraine without disclosing that work to the U.S. government. The Associated Press reported that Manafort’s firm received at least $1 million for his efforts, with payments coming from a bank in Belize. The U.S. Treasury Department, working with the FBI and CIA, also is looking at financial payments Manafort might have received through banks in Cyprus.

Further, AP reported that Manafort once secretly worked for a Russian billionaire close to Putin on a communications plan to influence politics, business dealings and news reports to Russia’s benefit.

Manafort’s spokesman has contended the work for the Ukrainian faction was “totally open and appropriate.” Manafort confirmed that he worked for Oleg Deripaska, the Russian billionaire, but said the work wasn’t pro-Russia in nature.

Mike Flynn

Trump’s initial national security adviser resigned after The Washington Post revealed that he misled Vice President Mike Pence about the nature and extent of his contacts with Russian Ambassador Sergey Kislyak. But the resignation merely opened the door for a deeper look at Flynn’s activities.

In 2015, Flynn was paid $45,000 for participating in a Moscow event honoring RT, Russia’s state-backed TV network, and another $22,000 for making two Russia-related speeches. Last December, Flynn met with Kislyak at Trump Tower, and on Dec. 29 made five phone calls to Kislyak — the same day President Barack Obama announced sanctions against Russia. Though he initially denied it, Flynn did discuss sanctions and could be vulnerable to blackmail, according to intelligence officials.

Jeff Sessions

The U.S. attorney general, during testimony under oath at his confirmation hearings, had twice told senators that he had no contact with any representative of the Russian government about Trump’s 2016 campaign before or after Election Day. Then The Washington Post reported that Sessions in fact met with Kislyak in July and September. Sessions recanted his previous statements and said he would recuse himself any campaign-related investigations.

Jared Kushner

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The president’s son-in-law, a key member of the administration’s inner circle, met during the election campaign with the Russian ambassador and a top-level Russian banker to discuss sanctions and a “backdoor channel” to Putin, according to several reports.

Kushner has agreed to testify before the Senate investigative committee. A Trump spokesman has said Kushner’s meetings were appropriate, given that he functioned as the person of contact with foreign officials during the campaign.

Donald Trump Jr.

The President’s eldest son is operating the family business. In a 2008 speech to a real-estate conference about emerging markets, he told attendees he had traveled to Russia six times in an 18-month period, saying that “buyers have been attracted to our projects.” The president himself has claimed he has no investments in Russia, but his son disclosed that Trump properties have Russian investors.

“And in terms of high-end product influx into the U.S., Russians make up a pretty disproportionate cross-section of a lot of our assets, say, in Dubai, and certainly with our project in SoHo and anywhere in New York,” Trump Jr. said, according to reports. “We see a lot of money pouring in from Russia.”

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Rex Tillerson

The secretary of state is the former CEO of ExxonMobil, where he developed deep ties with Russia. In 2011, his company signed a $500 billion joint venture with Rosneft, the Russian state oil company, to drill for oil on the arctic shelf and develop oil in Siberia. Soon after, Putin gave Tillerson the Russian “Order of Friendship” medal.

In his first weeks in office, Tillerson reportedly has stood for holding a hard line on the economic sanctions imposed on Russia after it invaded Crimea. Unlike Trump, Tillerson has said it’s “well-established” that Russians meddled in the 2016 U.S. election. He met with Putin in April; the two reportedly found little agreement on issues such as sanctions and Syria.

Roger Stone

A self-proclaimed political “dirty trickster” going back to the Nixon years, Stone has been a longtime Trump friend and adviser and former associate of Manafort’s. Along with Manafort, Flynn and Page, Stone is one of four people whose calls and contacts with Russian officials are under investigation by U.S. law enforcement, according to The New York Times.

Among other issues, Stone has acknowledged he has communicated with “Guccifer 2.0,” the online handle of a hacker who has claimed responsibility for hacking Democratic emails. American officials believe the Guccifer is a front for Russian security operatives. But Stone has said the contact was after the hacked emails were released.

“It’s only fair that I have a chance to respond 2 any smears or half truths about alleged “Collusion with Russians” from 2day’s Intel Hearing,” Stone wrote on Twitter.

Felix Sater

The Russian-born immigrant (who has a home in Port Washington, records show) once did prison time for stabbing a man in a bar, was convicted in a Mafia-related racketeering scheme in which he later became a witness for the prosecution, and has worked as an FBI informant. He also has worked with the Trump organization scouting real-estate deals for more than a decade.

He was part of several attempts by Trump to garner business deals in Russia, starting with a 2005 effort to build a “Trump Tower” in Moscow. In 2006, Sater reportedly accompanied Donald Trump Jr. and Ivanka Trump around the city to meet with potential business partners. The tower was never built. But later, Bayrock (which was headquartered in Trump Tower in New York) and Trump partnered to build properties in New York and Florida, financed by Russian and Kazakhstan money, according to a lawsuit, CNN reported.

In February, The New York Times reported that Sater and Michael D. Cohen, the president’s personal lawyer, helped deliver to the Trump administration a Russia-Ukraine peace settlement proposed by a pro-Russia Ukrainian lawmaker. Trump, in a 2013 deposition, reportedly said he barely knew Sater.

Sergey Kislyak

He has been Russia’s ambassador to the United States for nine years, an uncommonly long tenure. U.S. intelligence officials have called him a spy and a recruiter of spies, according to reports — which Russia denies.

Conversations with Kislyak before Trump taking office are what led to Flynn’s abrupt resignation and Sessions’ decision to recuse himself from the various probes of Russian meddling in the U.S. election. His meeting with Trump son-in-law and adviser Jared Kushner also is under scrutiny.