People close to Robert Mueller believe "something happened" to the former special counsel over the course of his two-year Russia investigation, according to a reporter.

The Washington Post's Carol Leonnig, who is the co-author of the new book A Very Stable Genius about President Trump, described on Tuesday how difficult it was for some of Mueller's close family friends to watch his shaky testimony before Congress last summer.

“Phil [Rucker] and I, my co-author, we are not medical professionals, but over and over again, John, we heard from people who are very close to Bob Mueller who found him a different person, a changed person, after two years of this investigation,” Leonnig told CNN host John Berman.

“They don’t know what that’s about,” Leonnig continued. “Some of them do and haven’t shared that with us. But they know that something happened. He’s a different person. He was stumbling over his words. You saw him in July in his testimony before Congress, there were people that I spoke to who are very, very good family friends of his who said, ‘I couldn’t watch the television anymore, I had to turn it off. It wasn’t the Bob I knew.'"

After a 22-month investigation, Mueller's team did not find sufficient evidence to establish a criminal conspiracy took place between the Trump campaign and the Russian government. He also declined to make a determination about whether Trump may have obstructed justice but did lay out 10 instances of possible obstruction that Democrats viewed as a road map to continue investigating and possibly seek impeachment. Trump is now facing two articles of impeachment stemming from his dealings with Ukraine.

Speculation about Mueller's health began to swirl when the former FBI director, known for being sharp and competent, appeared to have difficulty hearing lawmakers' questions and failed to recall key facts from his report when he testified before the House Judiciary and Intelligence committees in July.

In their book, Leonnig and Rucker wrote that when Attorney General William Barr met Mueller before his report was released, Mueller read from his notes, and his "hands shook as he held the paper. His voice was shaky, too." Barr and his deputy, Rod Rosenstein, "couldn’t help but worry about Mueller’s health.”

The Washington Post reported in July that Mueller's team denied rumors that the then-74-year-old's cognitive acuity was wavering, prompting Democrats to compel him to appear publicly. His resulting performance had some Democrats privately questioning whether Mueller was all there mentally.

“It was a painful reminder that age catches up to all of us,” one unnamed House Democrat who questioned Mueller said at the time. “Here you have this Vietnam hero and this post-Sept. 11 FBI director. You could tell he was having a hard time hearing, and it was like, ‘Ugh! This is not how I want him to be remembered.'"

Some critics, Berman said, believe Mueller just was not up to grappling with the "modern era of political warfare and may have got gamed out by the presidency."

“Many of our sources felt very strongly that Bob Mueller was playing a 1950s Boy Scout game," Leonnig said. "He was being an honorable icon and standard-bearer of the Department of Justice, whereas Bill Barr was playing in Trump’s 2000s, and he was messaging this very well. He was the first person to get out the message: Donald Trump’s been exonerated."

In early October, it was announced that Mueller had returned to work at the private law firm WilmerHale.