In a surprise news briefing on Thursday evening, Mr Biden insisted: “My memory is fine.”
He slammed a claim that he could not recollect when his son died, saying: “How the hell dare he raise that?”
The inquiry found Mr Biden “wilfully retained and disclosed” classified files, but decided not to charge him.
Department of Justice Special Counsel Robert Hur determined Mr Biden had improperly kept classified documents related to military and foreign policy in Afghanistan after serving as vice-president.
The scathing 345-page report, released earlier in the day, said the president’s memory had “significant limitations”.
Mr Hur interviewed the 81-year-old president over five hours as part of the inquiry.
The special counsel, a Republican appointed to the role by Biden attorney general Merrick Garland, said Mr Biden could not recall when he was vice-president (from 2009-2017), or “even within several years, when his son Beau died” (2015).
At Thursday night’s news conference, an emotional Mr Biden lashed out at the passages casting doubt on his recollection of events.
“Frankly, when I was asked the question I thought to myself, was none of their damn business,” he said.
He said he was “very occupied… in the middle of handling an international crisis” when he was interviewed by the special counsel from 8-9 October last year – just as the Israel-Gaza war erupted.
The inquiry also said Mr Biden had shared some of the sensitive material from hand-written notebooks with a ghostwriter for his memoir, a finding that the president denied from the podium.
The special counsel concluded it would be difficult to convict the president of improper handling of files because “at trial, Mr Biden would likely present himself to a jury, as he did during our interview of him, as a sympathetic, well-meaning, elderly man with a poor memory”.
Opinion polls indicate the president’s age is a concern for US voters ahead of November’s White House election. But Mr Biden told reporters on Thursday he was the most qualified candidate.
“I am well-meaning,” he said. “And am elderly. I know what the hell I’m doing. I put this country back on its feet.
“I don’t need his recommendation.”
However even as he sought to rebut reporters’ questions about his age and mental acuity, he inadvertently referred Egyptian leader Abdul Fattah al-Sisi as the “president of Mexico”.
Continue here: BBC
