What is this Magnitsky Act you speak of?

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.

“The law is clear,” said Juan Pachón, a spokesman for Senator Robert Menendez of New Jersey, the ranking Democrat on the committee. “It requires a determination and report in response to the letter we sent with Corker. The president has no discretion here. He’s either complying with the law or breaking it.”

The Trump administration imposed sanctions in November against 17 Saudis accused of being involved in the killing, but has refused to blame Crown Prince Mohammed bin Salman, a key ally and the country’s de facto ruler, despite a C.I.A. conclusion that the crown prince ordered it.

Secretary of State Mike Pompeo sent letters to the committee leaders describing actions taken against individuals without offering the determination of who was responsible as demanded by the lawmakers.

“I anticipate a more detailed briefing from the administration on this issue and look forward to working with them and the members of my committee in our ongoing effort to address the killing of Jamal Khashoggi,” said Senator Jim Risch, Republican of Idaho, who succeeded Mr. Corker as chairman.

The Saudi government on Friday rebutted a report that Prince Mohammed told a top aide in 2017 that he would use “a bullet” on Mr. Khashoggi if he did not return to the kingdom and cease his criticism of the Saudi government.