WASHINGTON (AP) — Relatives of the victims of the Sept. 11 attacks are calling for the Justice Department’s inspector general to investigate the FBI’s failure to produce certain pieces of evidence from its investigation.

The victims’ relatives regard that missing evidence as potentially crucial in proving their premise that the Saudi kingdom was complicit in the attacks.

That thesis forms the basis of a pending federal lawsuit in New York, though U.S. investigations have not conclusively established such a link.

Among the pieces of evidence, the families say the FBI has failed to produce are phone records of calls between conspirators in the attacks, records of witness interviews and a photograph of a Saudi diplomat with two of the hijackers outside a southern California mosque.