Flynn and his son have been eyed by Mueller's investigation in connection with a variety of issues, including the elder Flynn's contacts with the Russian ambassador, and the question of whether he tried to orchestrate the potentially illegal removal of a Turkish cleric from the United States in exchange for millions of dollars. That cleric has been a target of Turkish President Recep Tayyip Erdogan.
NBC News reported in early November that sources familiar with Mueller's probe had said the special counsel had assembled enough evidence against both Flynns to indict them on criminal charges.
In addition to Mueller's investigation, Flynn has faced inquiries from the House and Senate, as well as a separate Pentagon probe, for a variety of undisclosed ties to foreign governments.
The first undisclosed contact with a foreign government came in 2015 when Flynn traveled to Russia for the 10th-anniversary dinner for Russian broadcaster RT. Flynn told security clearance investigators that he was paid by American companies for the trip. In fact, he later acknowledged that he signed a $45,000 contract with the Russian state-funded network.
At the dinner, Flynn was seated next to Russian President Vladimir Putin and across from Sergey Ivanov, Putin's then-chief of staff, NBC News reported.
Flynn later found himself in trouble for failing to disclose his work during the Trump campaign for Turkish businessman Ekim Alptekin, which he later said in a filing "could be construed to have principally benefitted the Republic of Turkey."
In a strange twist, Mueller's agents were seeking details about a $15 million deal reportedly hatched in December between Flynn and senior Turkish ministers to deliver the cleric Fethullah Gulen from his exile in Pennsylvania to Turkish custody. Flynn's attorney has called the report "false."
Flynn's tenure in the White House ended after only 24 days because the former Defense Intelligence Agency chief misled Vice President Mike Pence about conversations Flynn had during the transition with Kislyak about U.S. sanctions on Russia. Flynn had previously met the ambassador for a brief meeting at Trump Tower that was also attended by senior Trump adviser Jared Kushner, the president's son-in-law.
Trump has stood by Flynn as the accusations piled up.
The day after Flynn resigned as national security adviser, Trump told then-FBI Director James Comey that Flynn was a "good guy" and suggested that the FBI director let the situation go, Comey told congressional investigators.
"I hope you can see your way clear to letting this go, to letting Flynn go," Trump told him, according to Comey's June testimony to the Senate intelligence committee.
The president has called the investigations into Flynn a "witch hunt."
Flynn and his attorneys had earlier sought immunity for charges related to the investigations.
In March, Flynn's attorney released a statement saying that "General Flynn certainly has a story to tell, and he very much wants to tell it, should the circumstances permit."
The Senate Intelligence Committee later rejected Flynn's immunity request, NBC News reported.
Obama warned Trump against hiring Flynn two days after Trump's election victory, according to The New York Times.