A source close to President Donald Trump said the latest developments surrounding his former national security advisor Michael Flynn are "very, very, very bad," according to NBC News.

Flynn pleaded guilty on Friday to lying to the FBI and agreed to cooperate with investigators working on special counsel Robert Mueller's Russia probe.



As part of Flynn's plea deal he agreed to "cooperate fully, truthfully, completely, and forthrightly" with the special counsel's office. His cooperation could include taking government-administered polygraph tests and participating in covert operations, according to the agreement.



Read more coverage about Michael Flynn:

Michael Flynn, Trump's ex-national security adviser, pleads guilty to lying to the FBI

Read: Michael Flynn's plea agreement

Stocks fall on report that Michael Flynn was directed by Trump to talk to Russians

Flynn is the first Trump White House official to be charged with a crime in connection to Mueller's investigation.

Flynn's plea is "a very big deal," said Gerald Lefcourt, a leading criminal defense attorney.