"You have to have intent in order to obstruct justice in the criminal sense," the source said, adding that "intent is hard to prove."

Comey will testify publicly before the Senate intelligence committee after Memorial Day, the panel's leaders announced Friday.

The central question at that blockbuster hearing will be whether Comey believed the President was trying to interfere with his investigation.

At a news conference Thursday, Trump angrily denied that he had asked Comey to end the investigation, which is now in the hands of new special counsel Robert Mueller. The President blasted the probe into Russia's involvement in the 2016 election and possible collusion with his campaign as a "witch hunt." 

Comey's view of Trump's intent in their conversations is nuanced, sources say. He initially believed that he could school the new President and White House in what was appropriate during their communications.