Hillary Clinton, questioned by federal investigators over whether she had been briefed on preserving government records before leaving the State Department, says she had suffered a concussion and could not recall every briefing she received.

Clinton, the Democratic Party's presidential candidate, raised the health scare during her 3-1/2-hour interview with the Federal Bureau of Investigation and Justice Department prosecutors on July 2, according to an FBI summary released on Friday.

The FBI also released other details of its investigation into her use of an unauthorised private email system while running the State Department in which it concluded she mishandled classified information but not in a way that warranted a criminal prosecution.

Clinton told investigators she could not recall getting any briefings on how to handle classified information or comply with laws governing the preservation of federal records, the summary of her interview shows.

"However, in December of 2012, Clinton suffered a concussion and then around the New Year had a blood clot," the FBI's summary said. "Based on her doctor's advice, she could only work at State for a few hours a day and could not recall every briefing she received."

The concussion was widely reported at the time, and Republicans have since used it to attack the 68-year-old candidate's health in a way her staff have said is unfounded.

Clinton, who is challenging Republican Donald Trump for the White House in the November 8 election, has been dogged for more than a year by the fallout from her decision to use an unauthorised private email account run from the basement of her Chappaqua, New York, home.

Republicans have repeatedly attacked Clinton over the issue, helping drive opinion polls that show many US voters doubt her trustworthiness.

Trump has repeatedly used the issue as part of his claims that Clinton is dishonest, and his campaign on Friday said the notes from the FBI report reinforced "her tremendously bad judgment and dishonesty."

Clinton has said that in hindsight she regretted using a private email system while secretary of state.

According to the report, Clinton told the FBI that she did not set up a private email server to sidestep the law requiring her to keep her business communications a matter of public record.

After her use of a private email system became public knowledge in March 2015, Clinton repeatedly said she did not use it to send or receive classified information. The government forbids handling such information outside secure channels.

The FBI has since concluded Clinton was wrong to say that: At least 81 email threads contained information that was classified at the time, although the final number may be more than 2,000, the report says. Some of the emails appear to include discussion of planned future attacks by unmanned US military drones, the FBI report shows.

The US government requires that military plans be classified.

Spokesmen for Clinton did not respond to questions about the concussion and other aspects of the FBI's summary, but released a statement welcoming the summary's release.

"While her use of a single email account was clearly a mistake and she has taken responsibility for it, these materials make clear why the Justice Department believed there was no basis to move forward with this case," Brian Fallon, a campaign spokesman, said in a statement.