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WASHINGTON — Federal investigators have gathered enough evidence to bring charges in their investigation of President Donald Trump's former national security adviser and his son as part of the probe into Russia's intervention in the 2016 election, according to multiple sources familiar with the investigation.

Michael T. Flynn, who was fired after just 24 days on the job, was one of the first Trump associates to come under scrutiny in the federal probe now led by Special Counsel Robert Mueller into possible collusion between Moscow and the Trump campaign.

Mueller is applying renewed pressure on Flynn following his indictment of Trump campaign chairman Paul Manafort, three sources familiar with the investigation told NBC News.

The investigators are speaking to multiple witnesses in coming days to gain more information surrounding Flynn's lobbying work, including whether he laundered money or lied to federal agents about his overseas contacts, according to three sources familiar with the investigation.

From left, retired Lt. Gen. Michael T. Flynn, his son Michael G. Flynn, and Boris Epshteyn, a spokesman for President-elect Donald Trump, board an elevator at Trump Tower in New York on Nov. 17, 2016. Carolyn Kaster / AP file

Mueller's team is also examining whether Flynn attempted to orchestrate the removal of a chief rival of Turkish President Recep Erdogan from the U.S. to Turkey in exchange for millions of dollars, two officials said.

A spokesperson for the special counsel had no comment.

Related: Mike Flynn's Son Is Subject of Federal Russia Investigation

Flynn's son, Michael G. Flynn, who worked closely with his father, accompanied him during the campaign and briefly worked on the presidential transition, could be indicted separately or at the same time as his father, according to three sources familiar with the investigation.

If the elder Flynn is willing to cooperate with investigators in order to help his son, two of the sources said, it could also change his own fate, potentially limiting any legal consequences.

The pressure on Flynn is the latest signal that Mueller is moving at a rapid and steady pace in his investigation. Last week, investigators unsealed indictments of Manafort and Manafort's business partner Rick Gates. They pleaded not guilty.

Michael G. Flynn at an RT event with his father Ret. Lt. Gen. Mike Flynn in Moscow in 2015. RT

Investigators also revealed Monday that former Trump campaign adviser George Papadopoulos had pleaded guilty to lying to federal officials and had been cooperating with Mueller's investigation.

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If the senior Flynn is charged, he would be the first current or former Trump administration official formally accused of criminal wrongdoing by the Mueller team.

So far, the probe has only ensnared campaign officials, and the White House has argued that the connection to the president is minimal. An indictment of the president's former national security adviser and his son would scramble that dynamic.

Related: Flynn, Manafort Are Key Figures in Mueller's Russia Probe

A former senior law enforcement official said that in the weeks after Trump's inauguration the FBI was asked to conduct a new review of Turkey's 2016 request to extradite Fethullah Gulen, an elderly Muslim cleric living in the U.S. whom President Erdogan blames for orchestrating a coup to overthrow him.

The FBI pushed back on the request because Turkey had supplied no additional information that could incriminate Gulen following a review of the case during the Obama administration, the official said. It is unclear whether the request to investigate Gulen came from Flynn or through the typical diplomatic channels at the State Department.

U.S.-based cleric Fethullah Gulen, whose followers Turkey blames for a failed coup, speaks to journalists at his home in Saylorsburg, Pennsylvania, on July 16, 2016. Greg Savoy / REUTERS TV / Reuters, file

The FBI is also investigating former CIA Director Jim Woolsey's account to The Wall Street Journal — which he confirmed to MSNBC — that Flynn and Turkish officials discussed a potential plan to forcibly remove Gulen from the country in September 2016, according to sources close to Woolsey, who say the former director has spoken to FBI agents working for Mueller about the matter.

Flynn was fired in February following public revelations that he had lied to Vice President Pence about his dealings with the Russian ambassador to the U.S., Sergey Kislyak.

Flynn's lawyer, Robert Kelner, declined to comment.

The younger Flynn's lawyer, Barry Coburn, declined to comment.

Father and Son

Both Flynns have for months been subjects of the Mueller investigation.

The elder Flynn, an Army lieutenant general, was pushed out as head of the Defense Intelligence Agency in 2014 and retired from the military. He then founded a lobbying firm, Flynn Intel Group, where his son worked closely with him. The younger Flynn was involved in the daily operations of his father's firm and functioned as his chief of staff. He often attended meetings with his father and would communicate with prospective clients.

The elder Flynn was paid $530,000 last year for work the Justice Department says benefited the government of Turkey. The elder Flynn did not register as a foreign lobbyist at the time, but did so retroactively this year. The issue has been part of Mueller's probe.

FBI Director Robert Mueller testifies before a Senate Judiciary Committee hearing ion oversight of the FBI on June 19, 2013 in Washington. Tom Williams / CQ-Roll Call file

His lawyer later said Flynn didn't need to register because his client was a Turkish businessman and not a government official, but had opted to do so retroactively.

According to Flynn's Justice Department filing, the Flynn Intel Group was hired to gather information about Gulen, and to produce a short film about its findings.

During the contract, which ended the day after Trump won the election, Flynn had at least one meeting, in September 2016, with Turkish officials, according to officials. Woolsey says that it included a discussion about kidnapping Gulen and flying him to Turkey.

Flynn also was paid some $35,000 in 2015 by Russian state television for a speech in Moscow at a gala where he sat next to Russian President Vladimir Putin. The younger Flynn accompanied him on that trip. The trip raised concerns among federal officials.

NBC News has reported that others under scrutiny by Mueller include Carter Page, a Trump campaign ally; Jared Kushner, the president's son-in-law and senior White House adviser; and the president's son, Donald Trump Jr. They have denied any collusion with Russia.

President Trump has denied any collusion with Russia during the campaign and has called the investigation a politically motivated witch hunt.

Kelner has declined to comment when asked if Flynn denies colluding with the Russian election interference effort.

Turkey has long demanded the U.S. extradite Gulen, saying he is considered a terrorist. Erdogan forcefully renewed that request after the attempted coup against him in July 2016. U.S. officials have said the Justice Department has not found sufficient evidence linking Gulen to the coup attempt despite the boxes of documents Turkey has submitted to the U.S. that Ankara says back up its claim.

Extradition requests are processed through the U.S. justice system and are not determined by the White House or other agencies.

Any quid-pro-quo deal such as the alleged agreement between Flynn and Turkey would be illegal, officials said.