Leading members of the US Republican Party have joined calls for a wide investigation into the former national security adviser’s links with Russia.
Michael Flynn quit on Monday over claims he discussed US sanctions with Russia before Donald Trump took office.
On Tuesday, a White House spokesman said Mr Trump knew weeks ago there were problems with the Russia phone calls.
But calls for an independent investigation have encountered a cold reaction from some senior Republicans.
The development came as the New York Times reported that phone records and intercepted calls show members of Mr Trump’s presidential campaign, as well as other Trump associates, “had repeated contacts with senior Russian intelligence officials in the year before the election”.
However, officials spoken to by the newspaper said they had not yet seen evidence that the Trump campaign colluded with Russia on the hacking of the Democratic National Committee or to influence the election.
He stood down over allegations he discussed US sanctions with a Russian envoy before Mr Trump took office.
It would have been illegal for Mr Flynn to conduct US diplomacy as a private citizen, before he was appointed as national security adviser.
The retired army lieutenant-general initially denied having discussed sanctions with Ambassador Sergei Kislyak. Vice-President Mike Pence publicly denied the allegations on his behalf.
Acting Attorney General Sally Yates had warned the White House about the contacts and that Mr. Flynn might be vulnerable to Russian blackmail on 26 January, said White House spokesman Sean Spicer.
President Trump had initially concluded that Mr Flynn’s actions did not violate any law, according to Mr. Spicer. (BBC)