The chairman of the House judiciary committee on Sunday said a report filed by the special counsel Robert Mueller could contain “proof of some very bad deeds” and warned that the entire document must be made available to Congress.

Mueller, a former director of the FBI, was appointed to investigate election tampering, alleged collusion between the Trump campaign and Russian operatives and related matters. He filed what has been described as a nearly 400-page report last month.

The attorney general, William Barr, is expected to release a redacted version of the Mueller report to Congress in the coming days. Portions will be withheld, Barr has said, including sensitive intelligence and information about allegations the special counsel declined to prosecute.

Last month, Barr released a letter saying Mueller had not established evidence of collusion between the Trump campaign and Russian operatives, but also saying Mueller had provided evidence of alleged obstruction of justice by Donald Trump while declining to recommend prosecution.

Speaking on Sunday, the judiciary committee chairman, Jerrold Nadler, a Democrat, said Barr had “showed his bias” in favor of Trump and accused Barr of “really acting as a personal agent of the president”.

“We also need to see the report,” said Nadler, “because it may be that Mueller decided not to prosecute for various reasons … but there still may have been proof of some very bad deeds and very bad motives, and we need to see that, and the public needs to see that.

“The fact of the matter is, we should see and judge for ourselves. And that’s for Congress to judge whether the president obstructed justice or not, and the public ultimately.”

Barr and the deputy attorney general, Rod Rosenstein, said they had determined that a criminal charge against Trump was not appropriate, based on evidence gathered by Mueller.

Rosenstein on Friday defended Barr’s handling of the process, saying the attorney general was “being as forthcoming as he can”.