The attorney general, William Barr, has said Robert Mueller “could have reached a decision” on whether Donald Trump is guilty of obstruction of justice – an implicit criticism of Mueller’s refusal to clear Trump of criminal activity.

In an interview with CBS News, Barr also contradicted the president over whether Obama-era officials committed treason. Trump has made repeated, vague, assertions that the FBI committed treason under Obama.

On Wednesday, in a move interpreted by some as an invitation for Congress to impeach Trump, Mueller said his team had deliberately not made a determination “as to whether the president did commit a crime”. Mueller said his decision not to formally state his opinion over whether Trump broke the law was down to justice department policy.

Speaking to CBS News, Barr said he disagreed with Mueller. “I personally felt he could’ve reached a decision,” Barr told CBS.

“The opinion says you cannot indict a president while he is in office, but he could’ve reached a decision as to whether it was criminal activity,” Barr said.

“But he had his reasons for not doing it, which he explained. And I am not going to, you know, argue about those reasons.”

Mueller’s report identified 10 incidents where Trump attempted to obstruct justice. The special counsel said justice department policy meant that a “president cannot be charged with a federal crime while he is in office”.

But, Mueller added: “If we had confidence that the president clearly did not commit a crime, we would have said that.”

Attorney General Barr highlights why he opened an investigation into the origins of the Russia investigation. Barr tells @JanCBS there's evidence that makes him believe senior government officials may have acted improperly to authorize surveillance of Pres. Trump's 2016 campaign. pic.twitter.com/CksxmmIu8i — CBS This Morning (@CBSThisMorning) May 31, 2019

Barr told CBS that the DoJ had “analysed the law and the facts” after receiving Mueller’s report. He determined that “as a matter of law many of the instances would not amount to obstruction”, he said.

In the interview Barr disagreed with Trump over whether Obama-era officials committed treason. Trump has claimed, without evidence, that his campaign was spied on during the 2016 election and claimed the Obama administration committed treasonous conduct as they investigated Russian election interference. The president has suggested unnamed people should serve “long jail sentences”.

Asked directly if he thought officials had committed treason, Barr said: “Not as a legal matter, no.

“Sometimes people can convince themselves that what they’re doing is in the higher interest and better good,” he said. “They don’t realize that what they’re doing is really antithetical to the democratic system that we have.”