As the 20th anniversary of the Sept. 11, 2001 terrorist attacks approaches, relatives of victims are pressing courts to answer what they see as unanswered questions about the Saudi government’s role in the attacks.

The questioning under oath of former Saudi officials in a lawsuit accusing Saudi Arabia of complicity took a significant step forward this year, but those depositions remain under seal, and the US has withheld a trove of other documents as too sensitive for disclosure.

The lack of information has aggravated families who have spent years attempting to prove that the Saudi government aided and abetted the attacks. Previous investigations have revealed links between Saudi nationals and some of the hijackers, but have not proven that the government was directly involved.

Lawyers for the victims intend to petition a judge to lift a protective order, allowing their clients access to secret government documents and testimony from key subjects interviewed over the last year. Though the plaintiffs’ lawyers are unable to discuss what they have learned from depositions, they maintain that the information they have gathered supports their premise of Saudi complicity.

The Saudi government has vehemently denied any involvement in the attacks. However, the question has long confounded investigators and is at the heart of a long-running Manhattan lawsuit on behalf of thousands of victims. The issue gained traction not only because 15 of the 19 hijackers were Saudi — as was the mastermind, Osama bin Laden — but also because there were suspicions they needed assistance navigating Western society given their limited experience in the United States.

Several Saudi entanglements have been detailed in public documents released over the last two decades, including by the 9/11 Commission, but no government complicity has been proven.

They depict how, in 2000, the first hijackers to arrive in the United States, Nawaf al-Hazmi and Khalid al-Mihdhar, were met and assisted by a Saudi national. According to investigators, the man who assisted them in finding and leasing an apartment in San Diego, Omar al-Bayoumi, had ties to the Saudi government. Just before meeting the hijackers, Bayoumi met with Fahad al-Thumairy, an accredited diplomat at the Saudi consulate in Los Angeles who, according to investigators, led an extremist faction at his mosque. Bayoumi and Thumairy left the United States several weeks before the attacks.

The 9/11 Commission, which compiled the most prominent account of the events leading up to the attacks, detailed those links but concluded that Bayoumi was a “unlikely candidate for clandestine involvement” with Islamic extremists. According to the report, while it was logical to consider Thumairy as a possible contact for the hijackers, investigators found no evidence that he actually assisted them. He has flatly denied it.

In general, the commission stated in 2004 that it found no evidence that the Saudi government or senior Saudi officials had funded al-Qaida, though it did note that money from Saudi-linked charities could have been diverted to the group.

The last chapter of a congressional report on the attacks was declassified in 2016. The document identified individuals who knew the hijackers after they arrived in the United States and assisted them in obtaining apartments, opening bank accounts, and connecting with mosques. It claimed that some of the hijackers had connections to and received support from people associated with the Saudi government, and that information obtained from FBI sources suggested that at least two of them were intelligence officers.

However, it did not reach a conclusion on complicity, stating that while the interactions could reveal evidence of Saudi government support for terrorism, there could also be more innocent explanations for the associations.

Families of 9/11 victims are hoping that similar allegations can be proven. They believe the entire story has not been revealed due to the US government’s unwillingness to provide a full accounting. Given Saudi Arabia’s role as a Middle East partner, any new evidence they uncover could be politically explosive.

The Saudi Embassy in Washington did not respond to a message seeking comment. The Saudi government’s lawyers declined to comment.

Another of the plaintiffs’ lawyers, Andrew Maloney, stated that they hope that, in addition to receiving compensation for their families, Saudi Arabia will accept responsibility and commit to combating terrorism.

The suit gained traction after a judge granted plaintiffs’ lawyers permission to conduct a limited fact-finding investigation in 2018.