He later referred to the investigation as a hoax during remarks at an event to recognize wounded soldiers. “This should never happen to another president again, this hoax,” he said.

As Mr. Barr defended the president, he said the special counsel’s considerations for whether the president obstructed justice must be taken in context.

“President Trump faced an unprecedented situation. As he entered into office and sought to perform his responsibilities as president, federal agents and prosecutors were scrutinizing his conduct before and after taking office and the conduct of some of his associates,” Mr. Barr said on Thursday morning, before his department publicly released the report.

“At the same time there was relentless speculation in the news media about the president’s personal culpability, yet as he said from the beginning, there was, in fact, no collusion.”

Mr. Barr’s views on executive power leave Mr. Trump with a nearly free hand.

The Constitution, according to Mr. Barr’s interpretation, does not consider it a crime for the president to wield his executive powers corruptly, for instance by firing a subordinate or by pardoning a loyal aide. It is an interpretation favorable to Mr. Trump.

“He alone is the executive branch,” Mr. Barr wrote in an unsolicited June 8, 2018 memo. “As such, he is the sole repository of all Executive powers conferred by the Constitution.” The document appeared to criticize Mr. Mueller’s inquiry into whether Mr. Trump had obstructed justice, and months later, when Mr. Barr received the Mueller report, which did not take a position on whether the president obstructed justice, Mr. Barr and Deputy Attorney General Rod J. Rosenstein concluded Mr. Trump did no such thing.

“There is substantial evidence to show that the president was frustrated and angered by his sincere belief that the investigation was undermining his presidency, propelled by his political opponents and fueled by illegal leaks,” Mr. Barr said on Thursday, in what appeared to be a defense of Mr. Trump’s relentless attacks on his political opponents, his own Justice Department and the special counsel’s investigation.