"I think it's really a sad thing he was treated so badly," Trump continued, calling Flynn a "wonderful man."
"I think in addition to that from intelligence, papers are being leaked … It's a criminal act."
Flynn resigned Monday after questions arose over his communication with the Russian ambassador to the United States during the transition period, when the Obama administration was pursuing new sanctions against Russia.
But White House press secretary Sean Spicer on Tuesday pointed to the "eroding" relationship between Flynn and Trump as what prompted the president to ask for Flynn's resignation.
The New York Times reported Tuesday night that there were "repeated contacts" between Trump aides and Russians during the campaign season but that there was no evidence that the campaign cooperated with the Russians to attempt to influence the election.
Still, Trump today dodged shouted questions about the matter.
"Mr. President, can you guarantee that nobody on your campaign had any contacts with the Russians?" ABC News' chief White House correspondent, Jonathan Karl, asked as Trump left the news conference. "Mr. President, any questions on Russia?"