WASHINGTON — Attorney General William Barr believes the Russia investigation that shadowed President Donald Trump for the first two years of his administration was started without any basis and amounted to an effort to “sabotage the presidency,” he said in an interview with Fox News Channel that aired Thursday.

Barr offered no support for his assertion that the FBI lacked a basis for opening the investigation and made no mention of the fact that the bureau began its probe after a Trump campaign adviser purported to have early knowledge that Russia had dirt on Democratic opponent Hillary Clinton.

Barr, who has appointed a U.S. Attorney to lead an investigation into the origins of the Russia probe, said the Justice Department has evidence there was “something far more troubling” than just mistakes during the investigation that eventually morphed into special counsel Robert Mueller’s probe.

“I think the president has every right to be frustrated because I think what happened to him was one of the greatest travesties in American history,” Barr said in the interview with Fox News Channel’s Laura Ingraham.

The attorney general said the FBI launched its counterintelligence investigation into ties between the Trump campaign and Russia “without any basis.”

“Even more concerning, actually, is what happened after the campaign, a whole pattern of events while he was president,” Barr said. “To sabotage the presidency, and I think that — or at least have the effect of sabotaging the presidency.”

The Justice Department’s inspector general found the FBI was justified in opening an investigation into ties between the Trump presidential campaign and Russia — to protect against a national security threat — and found the bureau didn’t act with political bias.

Trump and his supporters are counting on different conclusions from the separate investigation led by John Durham, the U.S. attorney Barr selected to examine the early days of the Russia probe. Durham’s investigation is ongoing, and Barr did not provide any evidence about what Durham has found so far.

Mueller concluded that the Russian government interfered in the 2016 election, but his investigation didn’t find sufficient evidence to establish a criminal conspiracy between Trump’s campaign and Russia. Mueller also examined about a dozen possible instances of obstruction of justice and has pointedly said he could not exonerate the president.

The inspector general’s report identified significant problems with applications to receive and renew warrants to monitor the communications of former Trump campaign aide Carter Page in 2016 and 2017. Investigators were concerned about Page’s ties to Russia but never charged him with any wrongdoing. Inspector General Michael Horowitz told senators the FBI failed to follow its own standards for accuracy and completeness when it sought warrants from the secretive Foreign Intelligence Surveillance Court to monitor Page’s communications.

The report detailed 17 errors and omissions during those wiretap applications, including failing to tell the court when questions were raised about the reliability of some of the information it had presented to receive the warrants. Those mistakes prompted internal changes within the FBI and spurred a congressional debate over whether the bureau’s surveillance tools should be reined in.

But Barr believes they were more than just mistakes, offering a personal view of the probe, a highly unusual move for a prosecutor in an ongoing investigation.

“My own view is that the evidence shows that we’re not dealing with just mistakes or sloppiness,” he said. “There is something far more troubling here, and we’re going to get to the bottom of it.

The FBI opened its investigation into ties between the Trump campaign and Russia on July 31, 2016.

By that point, Russian hackers had broken into the Clinton campaign and other Democratic email accounts and George Papadopoulos, a Trump campaign adviser, had boasted to a professor in London that he was aware that Russia had derogatory information on Clinton.

Though Trump and Barr have seized on errors made during the surveillance of Page, the investigation had already been underway for months by the time the first application was filed.