Special Counsel Robert Muller is looking into Flynn's lobbying work, including whether or not he lied to federal agents about his foreign contacts or laundered money, according to three sources familiar with the investigation.
White House officials have previously stressed that those charged to date were campaign officials who were not brought into government by Mr Trump.
If Flynn Sr.is indicted, he would be the first member of the Trump administration to face charges as part of the Russian Federation investigation, and it's not clear if President Trump would try to use his pardon powers to protect him in that situation.
In a week which began with Mueller's indictment on financial charges of former Trump campaign manager Paul Manafort and campaign aide Rick Gates - and news of a guilty plea and potential cooperation by a former foreign policy adviser, George Papadopoulos - White House and Republican party sources have sought to increase pressure on Mueller.
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Trump and his officials downplayed earlier arrests due to their work on the campaign rather than the administration, a charge for Flynn could would see that argument quashed.
It has been reported that Flynn's son, Michael G. Flynn, could also be indicted separately or at the same time as his father.
The anti-Trump media appears to have quite the connection to the investigation led by Special Counsel Bob Mueller. However, the very next day it was announced that Mueller planned to file the first set of charges in his investigation into Russia's interference in the 2016 election, in a sign the probe would likely continue well into 2018. It also means they are one step closer to President Trump. The retired army lieutenant general was sacked in February after less than a month on the job after it was revealed he had lied to Vice President Mike Pence about the extent of his dealings and communications with Russian Ambassador to the U.S. Sergey Kislyak. Another former Trump campaign aide, George Papadopoulos, has pleaded guilty to lying to the Federal Bureau of Investigation.
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