President Donald Trump asserted in his extraordinary letter firing James Comey that the former FBI director told him three times he was not under investigation.
If true, that claim would be a startling breach of protocol.
Former Justice Department officials and federal prosecutors said they doubted Mr Comey would have spoken to President Trump in those terms - or even offered any assurances at all.
Mr Comey has publicly confirmed the FBI is investigating possible contacts between the Trump presidential campaign and Russia.
President Trump offered no details about exactly what he was told, or when.
The White House declined to provide back-up or explanation for President Trump's assertion in the letter.