A federal lawsuit filed on behalf of over 3,500 victims’ relatives as well as first responders and survivors is pending in New York that is based on how Saudi Arabia was allegedly involved in the attacks where New York City’s twin towers came down. The victims’ relatives said the FBI’s investigation failed to locate certain evidence such as a photo of a Saudi diplomat alongside two plane hijackers in front of a mosque in southern California.

The FBI has said two case agents had no knowledge of the photograph and that “its search did not reveal” it, according to the complaint.

“The FBI has a duty to retain its 9/11 evidence and has sophisticated computer systems to maintain its files and run searches on them,” the complaint said. “A routine search must be able to locate critical documents from the most serious criminal investigation ever conducted by the FBI.”

For more reporting from the Associated Press, see below:

The victims’ relatives regard that missing evidence as potentially crucial in proving their premise that the Saudi kingdom was complicit in the attacks. U.S. investigations have not conclusively established such a link between the Saudi kingdom and the attacks.

Other pieces of evidence the families said the FBI has failed to produce are phone records of calls between conspirators in the attacks and records of witness interviews. The families had also asked the FBI to turn over a video they said shows a Saudi national hosting a party for two of the hijackers, but the FBI last year said it had looked for the recording and had so far been unable to find it.

The FBI in some instances said it had searched for but could not locate the evidence that the families had requested. The victims’ families said the photo evidence depicts a meeting outside the mosque with two of the hijackers, Nawaf al-Hazmi and Khalid al-Mihdhar.

The Justice Department revealed last month that the FBI had recently concluded an investigation that examined certain 9/11 hijackers and potential co-conspirators, and that it would now work to see if it could share information that it had previously determined could not be disclosed.

But, the families said Thursday, “thus far, no additional substantive material has been produced, and the FBI has not agreed to make a further effort to find its missing documents.”

The complaint urges Inspector General Michael Horowitz to “examine whether one or more FBI officials committed willful misconduct with intent to destroy or secrete evidence to avoid its disclosure.”

Many victims’ families have long alleged a closer connection between the Saudi kingdom and the hijackers than the U.S. government has acknowledged, and allege in a lawsuit in New York that Saudi officials gave critical support to the first two hijackers who arrived in America before the attacks.

Though the families filed a complaint with the inspector general’s office, there is no obligation on Horowitz to open an investigation.

Spokespeople for the inspector general and the FBI declined to comment Thursday.