WASHINGTON (Reuters) — Family members of victims of the Sept. 11, 2001, airplane attacks asked a U.S. government watchdog on Thursday to investigate their suspicions that the FBI lied about or destroyed evidence linking Saudi Arabia to the hijackers.

The request in a letter to Department of Justice Inspector General Michael Horowitz said, “Circumstances make it likely that one or more FBI officials committed willful misconduct with intent to destroy or secrete evidence to avoid its disclosure.”

The FBI declined to comment on the letter.

The latest in a series of requests over the 20 years since Islamist militants crashed civilian airliners in New York, Washington and Pennsylvania, 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 Horowitz 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 U.S. 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 Sept. 11 victim Bruce Eagleson, said in an interview.

Family members of victims have long sought U.S. government documents, including secret law enforcement and intelligence reports, related to whether Saudi Arabia aided or financed any of the 19 people associated with al-Qaida, the group given safe haven by the Taliban in Afghanistan at the time.

Fifteen of the 19 hijackers were from Saudi Arabia. A U.S. government commission found no evidence that Saudi Arabia directly funded al-Qaida. It left open whether individual Saudi officials might have.

Nearly 3,000 people were killed, including more than 2,600 at the World Trade Center, 125 at the Pentagon and 265 on the four planes.

The families of roughly 2,500 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 said in a court filing that it had decided to review earlier claims of privilege it had made about why 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 Aug. 9 in a statement welcoming the department’s commitment to a fresh review.

Reporting by Jan Wolfe and Mark Hosenball, additional reporting by Jonathan Landay; Editing by Scott Malone and Grant McCool