The request in a letter to Department of Justice said "circumstances make it likely that one or more FBI officials committed wilful misconduct with intent to destroy or secrete evidence to avoid its disclosure."

The Federal Bureau of Investigation declined to comment on the letter on Thursday.

This is the latest in a series of requests over the 20 years since Islamist militants crashed civilian airliners in New York, Washington and Pennsylvania, and seeks evidence including phone records and a videotape of a party in California attended by two of the hijackers more than a year before the attacks.

"Given the importance of the missing evidence at issue to the 9/11 investigation, as well as the repeated mishandling by the FBI of that evidence, an innocent explanation is not believable," said the letter, signed by about 3,500 people - families of victims, first responders and survivors.

It asked the department to investigate FBI statements made in response to a subpoena from the families that the agency "lost or is simply no longer able to find key evidence about the individuals who provided substantial support inside the US to the 9/11 hijackers."

Saudi Arabia has said it had no role in the hijacked plane attacks. The Saudi embassy in Washington did not immediately respond to a request for comment.

"Our government is either lying about the evidence it has or it is actively destroying it, and I don't know what's worse," Brett Eagleson, son of September 11 victim Bruce Eagleson, said in an interview.

Family members of victims have long sought US government documents related to whether Saudi Arabia aided or financed any of the 19 people involved in the attacks.

Fifteen of the them were from Saudi Arabia, but a government commission found no evidence they were directly funded by Saudi Arabia.

Nearly 3000 people were killed, including more than 2600 at the World Trade Center, 125 at the Pentagon, and 265 on the four planes.

The families of roughly 2500 of those killed, and more than 20,000 people who suffered injuries, businesses and various insurers, have sued Saudi Arabia seeking billions of dollars.

Last month, many families asked President Joe Biden to skip 20-year memorial events unless he declassified documents they contend will show Saudi Arabian leaders supported the attacks.

Three days later, the Justice Department announced it would why it had previously said it could not release some information requested by families.

"My administration is committed to ensuring the maximum degree of transparency under the law," Biden said on August 9 in a statement welcoming the department's commitment to a fresh review.

Australian Associated Press