Four years ago, Michael Flynn was the director of the Defense Intelligence Agency. One year ago, he was potentially facing jail time for misleading the FBI and Vice President Mike Pence about his conversations with the Russian ambassador to the United States. Today, he might be the linchpin to Special Counsel Robert Mueller’s investigation into collusion between President Donald Trump’s 2016 presidential campaign and the Russian government.

“When a special prosecutor, like Mueller, says someone has given ‘substantial assistance to an investigation,' that’s code for, 'He gave up everything,'” national security expert Juliette Kayyem told Boston Public Radio on Wednesday.

Read more: Michael Flynn Has Provided 'Substantial Assistance' In Russia Inquiry, Feds Say

In a court filing written by Mueller’s team in advance of Flynn’s Dec. 18 sentencing hearing for willfully making false statements to the FBI, Mueller said Flynn provided “first-hand insight” into the relationship between the Trump campaign and the Russian government, and praised him for his cooperation. He recommended Flynn receive little to no jail time for his help.

The praise for Flynn comes after a week when Mueller’s team accused former Trump campaign manager Paul Manafort of lying to federal investigators, Trump’s personal lawyer Michael Cohen entered into a second guilty plea for lying to Congress, and Trump advisor Roger Stoneinvoked the Fifth Amendment as a reason for not complying with the Senate Judiciary Committee’s requests for documents relevant to their own investigation into Russian collusion.

While much of Mueller’s investigation still remains unclear, one thing is for certain: The investigation has taken a serious personal, political, legal and financial toll on people close to Trump and his 2016 campaign.

As Kayyem succinctly put it, “If you come in and you lie, the guillotine is coming down.”