WASHINGTON—The Trump administration turned over a key piece of new information to lawyers for 9/11 victims’ families, a move that could cast light on long-disputed reports of Saudi government involvement in the terrorist attacks.

The Federal Bureau of Investigation provided the name of a Saudi official contained in a 2012 report that the bureau has long sought to shield from full public view. It declined to release any other information the families had sought, which could have further complicated already-tense relations between Washington and Riyadh.

The decision, revealed in a Thursday court filing, marks the latest turn in a long battle between the families and the FBI to get access to an unredacted copy of the report and other information. The families sought the document as part of a lawsuit against Saudi Arabia that accuses its government of helping coordinate the 2001 attacks.

Victims’ families had for months urged the government to provide the full report, telling President Trump in a letter recently that it would help them “finally learn the full truth and obtain justice from Saudi Arabia.”

The FBI, citing the “exceptional nature of the case” said it would provide only the name of the one Saudi official that the families had wanted most to obtain, but no other information. The name of the official won’t be made public, but would be given privately to lawyers for the victims’ families, according to the filing.