Donald Trump’s former national security adviser Michael Flynn just became the fourth — and potentially the most important — person charged in Special Counsel Bob Mueller’s investigation into whether the Trump administration colluded with Russia during and after the election.

Back in February 2016, Flynn, a retired three-star general, was one of the first high-level people to join Trump’s campaign, later making him a senior member of the transition team. And just 10 days after the election, Trump had already named Flynn as national security adviser, a role that would give him nearly unfettered access to White House secrets.

“Flynn is in the midst of every single meeting with [Jared] Kushner, the president, planning his Cabinet. He already knows he’s going to be the national security adviser,” said Bob Anderson, who served as executive assistant director of the FBI’s counterintelligence unit until December 2015. “He would have been involved in probably 98 percent of sensitive conversations and meetings that went on.”

Now, Flynn is cooperating with Mueller, according to a statement he made after pleading guilty to perjury on Friday for lying to the FBI, twice, about his interactions with Russian Ambassador Sergey Kislyak last December. The plea deal, however, suggests investigators have much more dirt on Flynn, like similar charges of money laundering and tax evasion as Trump’s former campaign manager Paul Manafort now faces.

READ: Here’s what Mike Flynn lied to the FBI about

All signs point to Mueller’s team letting Flynn off easy in exchange for testimony against higher-ranking members of the Trump campaign, transition team, or administration, experts told VICE News.

“It’s typical in a case like this that if the government anticipates using Flynn as a witness, the government would require him to admit to lying to anticipate cross-examination,” said Robert Litt, former general counsel for the director of National Intelligence.

Flynn also admitted to coordinating his contact with Kislyak with a “senior official” on Trump’s transition team at Mar-a-Lago, according to federal prosecutors. Multiple reports have so far corroborated that, and some have identified Trump’s son-in-law Jared Kushner as the instigator.

While the Trump administration originally denied anyone other than Flynn having contact with Russian officials, Kushner later said he had “perhaps four contacts” with the Russians himself — including a December 2016 meeting at Trump Tower, where Kushner reportedly proposed a back channel to Kislyak for the U.S. and Moscow to communicate. Flynn was there too.

Mueller will most certainly ask Flynn to go back through Kushner’s statements, as well as everyone else’s, especially considering his access to these conversations as an early member of the transition team and short-lived tenure as national security director, experts said.

As is, Flynn’s charge comes with a five-year maximum sentence in prison and a fine of up to $250,000. But when the DOJ offers a plea deal, the details aren’t always fully nailed down, according to Anderson. If Flynn doesn’t cooperate in the ways Mueller wants him to, he can always tack on some charges or threaten a harsher sentence as an incentive.

Flynn was also reportedly worried about Mueller’s scrutiny on his son, who served as his father’s chief of staff and was actively involved in their lobbying firm, the Flynn Intel Group. Through the firm, Flynn did almost half a million dollars worth of work for Turkey before the U.S. election and failed to tell the U.S. government — a potential violation of the Foreign Agents Registration Act — although he did later register. In his statement after pleading guilty on Friday, Flynn said he made a decision “in the best interest of my family and our country.”

Flynn’s even reportedly ready to testify against Trump, according to ABC’s Brian Ross as well as Fox News. If Flynn does — and depending on what he offers — the information could speak to obstruction of justice and even lead to impeachment. Despite the political fiasco any testimony against Trump would create, impeachment remains an unlikely option while Republicans control both houses of Congress.

“This looks like a pretty standard course of investigation by Mr. Mueller: Go after Mr. Flynn to get at the president,” Paul S. Ryan, vice president of policy and litigation at the Common Cause foundation, a civil litigation and anti-corruption watchdog group.

But that’s how these investigations always go, experts said. Investigators go after low-level people who “have a lot to lose and nothing to gain,” as Anderson put it, to reach more senior officials in an effort to secure the end goal of the investigation: proof of collusion with Russia, in this case. A former foreign policy adviser on Trump’s campaign George Papadopoulos also pleaded guilty to lying to the FBI about Russia, while Manafort and his associate Rick Gates, also charged in the Russia probe, pleaded not guilty and remain on house arrest.

READ: Mueller’s indictment of Manafort was written to terrify K Street

Aside from trying to reach Trump, Mueller may also be focusing on the Logan Act to ensnare other members of the Trump team, many of whom were still private citizens while Trump was president-elect, according to Ryan. The Logan Act prohibits private citizens’ from conducting foreign policy without permission from the government. And trying to undermine the Obama administration’s foreign policy before Trump officially took office could constitute a violation of this law. Still, no one has ever been convicted under the Logan Act. It’s been on the books since 1799.

Despite White House lawyer Ty Cobb’s attempt to distance the Trump administration from Flynn and frame him as a “former Obama administration official,” he left the previous administration about year early after a series of clashes about his leadership, including his loose relationship with the truth that staffers called “Flynn facts.” Obama even reportedly warned Trump not to rehire Flynn.

Flynn was sworn into the NSA on Jan. 22. Two days later, he lied to the FBI about discussing Russian sanctions with Kislyak.

After just 24 days as national security adviser, Flynn was forced to resign after it became clear that he had told the same lie to Vice President Mike Pence that he would eventually tell to the FBI — that he didn’t discuss U.S. sanctions against Russia with Kislyak. Former acting attorney general Sally Yates warned the Trump administration that Flynn’s lie left him vulnerable to blackmail.

Four days later, Trump fired Yates.