Washington (CNN) Michael Flynn on Friday became the first Trump administration official to be charged as part of special counsel Robert Mueller's investigation into possible collusion between the Russian government and the Trump campaign.

It was a stunning development not just for the White House, but in the career of a man who climbed through the military ranks to helm one of the United States' top intelligence agencies, the Defense Intelligence Agency, before serving as President Donald Trump's national security adviser.

Now, the former national security adviser finds himself charged with lying to the FBI -- becoming the first Trump administration official to face charges in Mueller's investigation.

Why is Flynn's indictment so significant?

Flynn is now the first person who worked inside the Trump White House to be charged with a crime. And it's Flynn's close relationship with Trump that makes the development particularly significant.

Flynn first began advising Trump in February 2016, and within a few months he began joining Trump at rallies across the country and soon became a near constant presence by the Republican nominee's side -- flying on Trump's plane and frequently introducing him at rallies.

Their bond was sealed during those months of campaigning and through their shared ideology on foreign policy and national security matters, from both advocating for closer ties with Russia to hawking hardline, often Islamophobic views on terrorism.

It all led to Flynn being named national security adviser to the President soon after the election.

So Trump and Flynn grew close. Why did he only last 24 days as national security adviser?

Flynn was forced to resign after less than a month after he misled Vice President Mike Pence and then-chief of staff Reince Priebus about his conversations with then-Russian Ambassador to the US Sergey Kislyak.

Flynn had told them -- and the public -- that he did not discuss US sanctions against Russia during a conversation with Kislyak in December 2016 -- before Trump's was inaugurated as President. But he had.

Flynn's conversations with Kislyak would become a focal point of the FBI's investigation into Flynn -- and raise questions about the Trump campaign's potential interference with that investigation.

That provoked a domino effect, with FBI Director James Comey later alleging that Trump asked him to drop the investigation into Flynn.

" 'I hope you can see your way clear to letting this go, to letting Flynn go. He is a good guy. I hope you can let this go,' " Comey wrote in contemporaneous notes, quoting Trump

And on Friday, the federal charges against Flynn again came back to Flynn's conversation with the Russian ambassador -- with the indictment alleging Flynn made false statements to the FBI about two conversations with Kislyak.

Trump remained close to Flynn even after he was pushed out?

It looks that way. Yahoo news reported that Flynn told a group of friends in April that he and the President were still in touch and that Trump had sent him a message to "stay strong."

In public comments in the wake of Flynn's resignation, Trump has continued to praise Flynn, calling him a "very good person" in a May interview.

It's not clear when Trump and Flynn last spoke.

Before he got in trouble for his conversations with the Russian ambassador, were there any signs Flynn might be trouble for the President?

Yes.

Flynn's legal issues stem in part from foreign payments he received after he started his own consulting firm.

Flynn founded the Flynn Intel Group after he retired from the military in 2014.

The FBI began investigating Flynn in the fall of 2016 for secretly working during the presidential campaign as an unregistered lobbyist for Turkey, an investigation he disclosed to the Trump transition team before joining the administration as Trump's national security adviser.

As part of his pro-Turkey lobbying, Flynn and his colleague held meetings in September and during the Trump transition with Turkish representatives. According to The Wall Street Journal , Flynn and his son were offered as much as $15 million to forcibly remove a Turkish cleric wanted by Turkey from the United States.

CNN's legal analysts say the alleged plan to kidnap Fethullah Gulen and subvert the US legal process would directly violate the US criminal code and could carry a punishment of as much as 20 years in federal prison. Lawyers for Flynn said the allegations "ranging from kidnapping to bribery" are completely false.

Flynn's lobbying firm was paid $530,000 by Inovo BV, a company owned by Turkish businessman Ekim Alptekin, during the final months of the US presidential campaign. The Flynn Intel Group, which is no longer active, was tasked with producing an anti-Gulen documentary, though the final product never aired.

Is that it?

And he failed to disclose those payments in his initial financial disclosure form filed in February.

Among the payouts, Flynn received $33,000 of a $45,000 speaking fee for a 2015 speech at a Moscow event hosted by RT, where he sat at the same table as Russian President Vladimir Putin.

Flynn's presence at the gala celebrating RT's 10th anniversary raised eyebrows among his critics. The US intelligence community said earlier this year that the Kremlin uses RT to push propaganda on American audiences, and that the English-language channel was involved in the effort to interfere in the election.