Veteran reporter Bob Woodward on Friday told radio host Hugh Hewitt he looked "hard" for evidence of collusion between President Donald Trump and Russia but didn't find anything.

But he still thinks the special counsel Robert Mueller has "something" on the president.

Woodward suggested Mueller possibly has "a secret witness or somebody who has changed their testimony."

Trump has firmly rejected allegations of collusion between his campaign and Russia in the 2016 US presidential election and often refers to the Mueller probe as a "witch hunt."

Veteran reporter Bob Woodward on Friday told conservative radio host Hugh Hewitt he looked "hard" for evidence of collusion between President Donald Trump and Russia but didn't find anything.

But he still thinks the special counsel Robert Mueller has "something" on the president.

Hewitt had Woodward on his show to discuss the reporter's new book, "Fear," which portrays the Trump administration as mired in consistent turmoil. The book paints a picture that suggests Trump's advisers don't respect him and often work against his impulsive nature.

At one point during the show, Hewitt began discussing Trump's former lawyer, John Dowd, who has strongly urged Trump not to sit down for an interview with the special counsel. This discussion led to the subject of collusion.

"Did you, Bob Woodward, hear anything in your research in your interviews that sounded like espionage or collusion?" Hewitt asked Woodward.

"I did not, and of course, I looked for it, looked for it hard. And so you know, there we are," Woodward said.

The reporter then added that he believes Mueller likely has "something" on Trump.

"We’re going to see what Mueller has, and Dowd may be right," Woodward said. "He has something that Dowd and the president don’t know about, a secret witness or somebody who has changed their testimony. As you know, that often happens, and that can break open or turn a case."

Trump has firmly rejected allegations of collusion between his campaign and Russia in the 2016 US presidential election and often refers to the Mueller probe as a "witch hunt."