Just ahead of his departure to speak to graduates of an FBI training program, U.S. President Donald Trump took the opportunity to further impugn the agency.

"It's a shame what's happened with the FBI," he said, calling its handling of the Hillary Clinton email investigation "really disgraceful."

He promised to "rebuild" the agency, having said in a series of tweets earlier this month that its reputation was "in tatters" and "the worst in history."

Trump has repeatedly dismissed claims by the FBI and other law enforcement agencies of Russian meddling in the election. He also called the FBI's investigation into possible ties between his campaign and Russia a "witch hunt."

"There is absolutely no collusion — that has been proven," he said to reporters Friday as he left the White House, bound for the FBI campus in Quantico, Va.

"I didn't make a phone call to Russia. I had nothing to do with Russia, everybody knows it. That was a Democrat hoax. That was an excuse for losing the election. So now even the Democrats admit there's no collusion."

It wasn't clear what admission he was referring to.

Trump alluded to newly revealed edits to former FBI director James Comey's 2016 statement on the Clinton probe: "It is very sad when you look at those documents. How they've done that is really, really disgraceful, and you have a lot of really angry people who are seeing it."

He said the recent revelations prove his claim that the Clinton investigation was "rigged."

Later Friday, Trump told the graduating class of the FBI National Academy that they are heroes, and the country has to do a better job of showing them the respect they deserve — including a call for the death penalty for anyone who kills a law enforcement officer.

"Deliver a message to your fellow officers: the president of the United States has your back 100 per cent," he said.

"We will protect those who protect us. And we believe criminals who kill police officers should get the death penalty," he said to graduates of the 10-week National Academy program.

The academy is an invitation-only "professional course of study" for U.S. and international law enforcement, according to the agency's website.