Special Counsel Robert Mueller's sprawling investigation has spawned a new guessing game in Washington centered on retired Lt. Gen. Michael Flynn, President Trump's former national security adviser.

Despite what some experts say is almost-indisputable evidence of wrongdoing - and reports that Mueller has gathered enough evidence to indict - no charges have been filed against the onetime intelligence officer.

The question for many is whether - or when - Flynn will be indicted by Mueller. Others speculate that he might already be cooperating with the special counsel's investigation.

Flynn, who stepped down after a mere 24 days in the White House, is seen as vulnerable for a number of reasons.

According to multiple outlets, he is under investigation for an alleged quid pro quo with the Turkish government, in which Flynn would have been paid millions of dollars in exchange for the extradition of a Muslim cleric living in the U.S.

Federal records show that Flynn did not register $530,000 he was paid during the 2016 campaign for work he did that the Justice Department said principally benefitted Turkey - a potential violation of the Foreign Agents Registration Act.

Heightening the personal drama, Flynn's son is also thought to be a focus of the Mueller probe - a potential tool that the special counsel could use to put pressure on Flynn.

Mueller's surprise announcement of the first charges in the investigation has intensified the speculation.

In back-to-back bombshells in late October, Mueller indicted former Trump campaign chairman Paul Manafort on nine criminal counts, including money laundering and tax fraud, and announced that another Trump aide, George Papadopoulos, had pleaded guilty to lying to federal investigators about his contacts with Russians.

Some legal experts said Mueller's decision to move forward with an indictment of Manafort is a signal that he is trying to "flip" him - or convince him to act as an informant on the inner workings of the Trump campaign.

And court documents suggest that Papadopoulos is already acting as a cooperative witness, leading to speculation that Mueller may be seeking to leverage his knowledge to build a case against more high-ranking officials from the campaign - or potentially even against Trump himself.

A similar technique could be at play with Flynn, who had far more intimate access to the campaign than Papadopoulos, some legal experts say.

According to CNN, Flynn has expressed concerns about the potential legal exposure his son faces.

"The obvious leverage that Mueller has over Flynn is that he could indict Flynn's son," tweeted Renato Mariotti, a former federal prosecutor who is running for Illinois attorney general as a Democrat. "Flynn could cooperate on his son's behalf."

This is a common technique used by prosecutors, legal experts say: Build a case around a family member or spouse to put pressure on a witness to "flip."

"You could imagine the same thing happening here, unless Trump intervenes with pardons," Mariotti tweeted in a separate thread.

Jack Sharman, who was special counsel to Congress during the Whitewater investigations into Bill and Hillary Clinton's real estate affairs, cautioned against reading too much into the absence of charges against Flynn.

"I would not read too much into that either way," he told The Hill. "You could have a scenario, as in any white collar or corruption case, where there are contacts back and forth between defense counsel and prosecutors and that may or may not develop into a proffer agreement."

"I would not read a conclusion into the fact that nothing has happened yet with Gen. Flynn."

Flynn has previously offered to cooperate with federal and congressional investigators in exchange for immunity - but found no takers, at least on the Hill.

"General Flynn certainly has a story to tell, and he very much wants to tell it, should the circumstances permit," his lawyer said in a March statement.

But the former White House official is a potentially valuable witness to Mueller, whose broad mandate includes investigating any coordination between the Trump campaign and Russia to swing the 2016 election.

Flynn served as an adviser to Trump's campaign beginning in February of 2016 and was at one point under consideration to be Trump's running mate. Trump ultimately chose Indiana Governor Mike Pence, but shortly after the election tapped Flynn as national security adviser - despite a reported warning from then-President Obama.

Flynn was forced to resign in February after it came to light that he had misled Pence about the nature of pre-inauguration phone conversations he had with the Russian ambassador.

Democrats - and some Republicans - say that the calls were almost certainly a violation of the Logan Act, an obscure and likely unenforceable 1799 law prohibiting private citizens from engaging in foreign policy.

The White House has sought to distance itself from Flynn since his ouster, although Trump continued to defend his former adviser. He asked former FBI Director James Comey to "let go" of the investigation into Flynn and in the past has reportedly told advisers he believed firing Flynn was a mistake.

But the latest allegations against Flynn potentially constitute a number of very serious federal crimes - including conspiracy, bribery and possibly kidnapping.

Investigators are looking into an alleged meeting in New York in December between Flynn and Turkish officials to discuss forcibly removing the cleric, Fethullah Gülen, and delivering him to Turkey in return for as much as $15 million, The Wall Street Journal reported this week.

Turkish President Recep Tayyip Erdoğan sees Gülen, who is living legally in the U.S., as a political enemy and has pressed the U.S. to extradite him.

Flynn had already been tapped as national security adviser at the time. The meeting was a follow-up to a September discussion attended by former CIA Director James Woolsey.

That meeting made Woolsey, then acting as an adviser to the campaign, so uncomfortable that he informed then-Vice President Joe Biden through an intermediary.

The removal was discussed as "a covert step in the dead of night to whisk this guy away," Woolsey said.

Investigators are also looking at how Flynn's son, Michael Flynn Jr., may have potentially been involved in the alleged deal, according to NBC News. Flynn Jr. worked closely with his father, acting as chief of staff for Flynn's lobbying firm.

The younger Flynn has denied any wrongdoing.

"The disappointment on your faces when I don't go to jail will be worth all your harassment," he tweeted last weekend.