U.S. Attorney General William Barr's letter to U.S. lawmakers stating that the investigation by Special Counsel Robert Mueller has been concluded and that Mueller has submitted his report to the Attorney General is seen in Washington, U.S. March 22, 2019. REUTERS/Jim Bourg

WASHINGTON (Reuters) - U.S. members of Congress are expected to receive a letter from Attorney General William Barr within the hour, a Department of Justice official said on Sunday, as lawmakers await a summary of Special Counsel Robert Mueller’s report on possible collusion between President Donald Trump’s campaign and Russia.