President-elect Donald Trump made another controversial Cabinet appointment on Friday, selecting retired Lt. Gen. Michael Flynn to serve as his national security adviser.

Flynn, a registered Democrat who headed the Defense Intelligence Agency from 2012 to 2014 before he was reportedly forced out by his peers, was one of Trump's earliest and most prominent supporters in the foreign-policy realm. But he has come under scrutiny for some of his business ties, his views toward Russia, and — like Trump — his Twitter account, where he often promoted conspiracy theories during Trump's campaign.

Among them: a fake news story connecting Democratic vice-presidential nominee Tim Kaine to the Palestinian terrorist organization Hamas, a story falsely suggesting Democrat Hillary Clinton was implicated in "sex crimes," and a meme making unsubstantiated claims about prominent journalists colluding with Clinton.

Flynn also tweeted in February that "fear of Muslims is RATIONAL," including a link to a YouTube video that claims the religion of Islam wants "80% of people enslaved or exterminated."

'His vision was seen as disruptive'

By most accounts, Flynn had a successful military career, and President Barack Obama appointed him to lead the Defense Intelligence Agency in 2012.

He was reportedly forced out in 2014, however, because "his vision" for the agency "was seen as disruptive," a former Pentagon official who worked closely with Flynn told The Washington Post at the time.

Flynn, who served in Afghanistan and Iraq from 2001 to 2012, has said he was fired from the DIA because he spoke out against "radical Islam," which he has called "a cancer" and made it his mission to eradicate.

But the former official told The Post at the time that the friction was more because Flynn was too hawkish. The official said Flynn wanted to push agents "up and out of their cubicles into the field to support war fighters or high-intensity operations."

Michael Flynn. Lauren Victoria Burke/AP Photo

Flynn has also been heavily criticized for attending a gala last year in Moscow celebrating the 10th anniversary of the state-sponsored news agency Russia Today. Flynn has appeared on and been interviewed by Russia Today more than once, and he has said he "absolutely agrees" that the US and Russia need to work together to defeat ISIS.

Russia intervened in Syria on behalf of Syrian president Bashar Assad in September 2015, nearly five years into the country's brutal civil war. Though Russia touted its involvement as an anti-ISIS operation, the vast majority of its airstrikes over the past year have targeted rebels fighting Assad in western Syria who have no association with the jihadist group.

Rep. Adam Schiff, of California, the ranking member of the House Intelligence Committee, said in a statement Friday that he was "deeply concerned" about Flynn's views on Russia, which, he said, "over the last twelve months have demonstrated the same fondness for the autocratic and belligerent Kremlin which animate President-elect Trump's praise of Vladimir Putin."

Flynn Intel Group

A consulting firm Flynn founded with his son after he retired, Flynn Intel Group — a registered lobbying organization — also has ties to Turkey's increasingly authoritarian president, Recep Tayyip Erdogan, through one of its clients, Kamil Ekim Alptekin.

Alptekin chairs the Turkish-American Business Council, whose members are chosen by Turkish government officials, and helped organize Erdogan's visit to Washington earlier this year.

Alptekin told The Intercept on Thursday that his company, Inovo BV, which is based in the Netherlands, paid Flynn's firm thousands of dollars for analysis beginning in August — the same month Flynn began sitting in on classified intelligence briefings with Trump, Yahoo News reported on Thursday.

Alptekin insisted that Flynn Intel Group had not been lobbying for his company, but the firm is registered as a lobbyist for Inovo BV, as The Intercept reported.

Turkish President Recep Tayyip Erdogan, left, listening to remarks by US President Barack Obama after a bilateral meeting alongside the G-20 Summit, in Hangzhou, China, on September 4. Jonathan Ernst/Reuters

Flynn wrote an op-ed article last week supporting the extradition of Pennsylvania-based cleric Fethullah Gulen, whom Erdogan has accused of inciting a coup attempt against his government in July. The Obama administration has denied the extradition requests, saying the Turkish government has not presented evidence showing Gulen to be complicit in the failed coup.

Erdogan's harsh crackdown on dissidents and journalists inside Turkey, as well as his renewed war with Kurdish separatists in Turkey's southeast, has also led to chilled relations between Washington and Ankara.

"General Flynn's uncritical acceptance of the Turkish crackdown on dissent, and his call for the extradition of the cleric Gulen without seeing any evidence of complicity in the aborted coup is also worrying," Schiff said in his statement.

"While Turkey is an important NATO ally, we cannot blind ourselves to its increasingly authoritarian character and the bonds it has been forming with Russia and Iran," he added.

In his own statement released Thursday, Flynn said, "If I return to government service, my relationship with my company will be severed in accordance with the policy announced by President-elect Trump."

Trump's transition team announced earlier this week that all registered lobbyists had been purged from its ranks.