By Tom Crann and Cody Nelson

Minnesota U.S. Sen. Amy Klobuchar said the U.S. should launch several investigations of Michael Flynn and his connections to Russia.

Flynn, Trump’s top national security aide for 24 days, reportedly resigned on Monday due to what White House press secretary Sean Spicer described as a “gradual erosion of trust.”

That resignation wasn’t the first Russia-related ouster of a top Trump confidant. Paul Manafort, Trump’s former campaign manager, was forced to resign last summer after his ties to pro-Kremlin Ukrainian leaders were revealed.

Both instances signal that there could be more to the White House’s involvement with Russia, especially after investigations by U.S. intelligence agencies have concluded Russia interfered in U.S. elections, said Klobuchar, a Democrat.

“It just puts us in a very vulnerable position,” Klobuchar said. “This is about the fundamental concept of our democracy.”

A U.S. official told The Associated Press that Flynn was frequently in contact with Russian Ambassador Sergey Kislyak on the day the Obama administration sanctioned Russia for hacking in the presidential election, as well as other times while the Trump administration transitioned into the White House.

Flynn did not disclose those talks to Vice President Mike Pence.

Klobuchar not only has concerns about Flynn’s conversations being kept under wraps, she said, but “we have a huge concern about the underlying problem, which is why are we kowtowing to Russia?”

Senate Majority Leader Mitch McConnell, a Republican, said it’s “highly likely” the Senate intelligence committee will probe Flynn’s contacts with Russia.

The Kremlin has downplayed Flynn’s dismissal.