Michael Flynn will be a key figure in Robert Mueller's special Russia probe

— -- President Trump’s short-lived National Security Advisor, Lt. Gen. Michael Flynn, has emerged as a central figure in ongoing investigation into Russia’s interference in the 2016 presidential election, which Former FBI Director Robert Mueller was appointed to oversee as special counsel on Wednesday.

ABC News has confirmed that a federal grand jury has issued subpoenas to Flynn’s private-sector associates and the U.S. Senate Select Committee on Intelligence has requested documents detailing Flynn’s foreign contacts, his business clients, and his communications with the Russian ambassador.

This week brought word that former FBI Director James Comey kept detailed notes suggesting that, one day after Flynn was forced to resign for lying to Vice President Mike Pence, President Trump urged Comey during a February Oval Office meeting to “see your way clear to … letting Flynn go.” Trump aides have disputed Comey’s account, but if accurate, the Comey memos suggest Trump was growing increasingly uneasy about the investigations into Flynn’s conduct.

Flynn is a life-long military officer with a decorated career, much of it in operational roles overseeing the United States Army’s intelligence-gathering arm. He served in Afghanistan and Iraq, rising swiftly up the ranks. He commanded the 111th Military Intelligence Brigade, was director of intelligence for Joint Special Operations Command, and he was director of intelligence of the United States Central Command. He oversaw the tracking and killing Abu Musab al-Zarqawi, the Emir of Al Qaeda in Iraq, which eventually became the Islamic State. In 2011, he took over the Defense Intelligence Agency.

Richard Frankel, a retired FBI agent and ABC News contributor, said Flynn had a reputation for sharp elbows and found himself “in conflict with others who were not happy with his rise up the ranks, especially when he became director of the DIA.” In 2014, Flynn retired as a three-star general, reportedly after being forced out the of the senior intel post. Those close to him say the anger he harbored towards the Obama administration drove him to jump into campaign politics.

After appearing with several Republican candidates in the Fall of 2015, he moved squarely behind Trump, eventually becoming a vocal champion at the candidate’s campaign rallies and on television. When Trump was elected, he approached Flynn about becoming National Security Adviser. According to Michael Ledeen, a historian and neoconservative political analyst who cowrote the book "Field of Fight" with Flynn about battlefield intelligence, Flynn was initially reluctant.

“Trump wanted him although Flynn [initially] said he didn't want the job,” Ledeen told ABC News. “He's a very talented man. He revolutionized U.S. military battlefield intelligence and was attempting to do the same thing at DIA when he was fired for telling the truth under oath.”

But Flynn accepted, and many believe his new post as the senior national security adviser to President Trump, made him a target. One Flynn confidant told ABC News the probing into Flynn’s business background started even before the 2016 election.

Before rejoining the government, Flynn founded a consulting firm, the Flynn Intel Group, and accepted several paid speaking engagements. The first hints of controversy came when photos emerged from a December 2015 trip Flynn took to Moscow showing him seated within arm's reach of Russian President Vladimir Putin. Russia's state-owned TV network RT paid Flynn $33,750 to attend.

At the same time, his firm began doing work for a firm with ties to the Turkish government. The former Central Intelligence Agency Director James Woolsey had served as an unpaid advisor to the group but told reporters he dropped out after attending a September 2016 meeting. Woolsey grew concerned about the work the firm was considering – including a brainstorming session in which the group entertained whisking a critic of the Turkish government out of the United States in the dark of night.

Perhaps more concerning, the firm had never registered with the Justice Department as a foreign agent for Turkey. Failing to register as a foreign agent can be a crime in some circumstances. In March, after it became clear the FBI was looking into possible registration violations, an attorney for Flynn made an effort to remedy the problem, filing papers detailing $530,000 worth of lobbying work Flynn had done for his Turkish client prior to Election Day.

But Flynn’s White House job became imperiled shortly after Justice Department lawyers began reading transcripts of recorded calls between Flynn and the Russian ambassador to the United States, Sergei Kislyak. Flynn had told Vice President Mike Pence that he had not discussed economic sanctions with Kislyak, but the transcripts revealed that was not true.

Sally Yates, who was serving as the acting attorney general, later told Congress that she was concerned that Flynn had lied to the vice president about his Russian contacts, and the Russians knew it. That, she argued, made him vulnerable to blackmail.

“We wanted to tell the White House as quickly as possible,” Yates testified in May. “To state the obvious: You don’t want your national security adviser compromised with the Russians.”

Flynn has not spoken publicly since his resignation, but some close to him told ABC News he has grown uneasy watching Yates level accusations through the media. Frankel, the retired FBI agent and a friend of Flynn’s, told ABC News he did not agree with Yates’s conclusion that Flynn had been compromised by the Russians.

“Conversations between the National Security Advisor and a foreign official – Russian or otherwise -- that is not something that exposes him to blackmail,” Frankel said. “It’s done all the time. If someone tried to blackmail you, you would just call your boss and explain it. Lots of senior government officials speak with their counterparts around the world.”

In the days after his resignation, Flynn’s lawyer floated the idea that he would meet with Senate investigators in exchange for an offer of immunity, but political leaders called that idea premature. His legal team and supporters have continued to maintain that Flynn has done nothing wrong, and that the overheated political climate is fueling the ongoing probes into his conduct, but questions have persisted.

“We in Congress need to know who authorized his actions, permitted them and continued to let him have access to our most sensitive national security information despite knowing these risks,” said Rep. Elijah Cummings of Maryland, the ranking Democrat on the House Oversight and Government Affairs Committee, in February. “We need to know who else within the White House is a current and ongoing risk to our national security.”

This week, at least, it appears that Flynn’s legal troubles have taken a back seat, as President Trump’s purported efforts to protect Flynn from scrutiny have landed Trump, once again, in the spotlight.