WASHINGTON — President Trump refused to provide Congress a report on Friday determining who killed the journalist Jamal Khashoggi, defying a demand by lawmakers intent on establishing whether the crown prince of Saudi Arabia was behind the grisly assassination.

Mr. Trump effectively bypassed a deadline set by law as his administration argued that Congress could not impose its will on the president. Critics charged that he was seeking to cover up Saudi complicity in the death of Mr. Khashoggi, an American resident and a columnist for The Washington Post.

“Consistent with the previous administration’s position and the constitutional separation of powers, the president maintains his discretion to decline to act on congressional committee requests when appropriate,” the Trump administration said in a statement. The statement said the administration had taken action against the killers and would consult with Congress.

But Democrats said Mr. Trump was violating a law known as the Magnitsky Act. It required him to respond 120 days after a request submitted in the fall by committee leaders — including Senator Bob Corker, Republican of Tennessee and then the chairman of the Senate Foreign Relations Committee — a period that expired Friday.