WASHINGTON: The FBI released hundreds of pages of newly declassified documents on Wednesday about its long effort to explore connections between the Saudi government and the Sept 11 attacks, revealing the scope of a strenuous but ultimately fruitless investigation whose outcome many question to this day.

Agents for years investigated support given to several of the hijackers upon their arrival in the US, focusing in particular on whether three Saudi nationals including a Saudi Embassy official in Washington had advance knowledge of the attacks.

Ultimately, investigators found insufficient evidence to charge any of the three with illegally supporting the hijackers, according to an FBI memo from May that closed out the probe and was among the more than 700 pages released on Wednesday. The FBI noted in the memo that Al Qaeda compartmentalised the roles within its major attacks and did not make the attack plans known in advance to others for fear of word getting out.

Specifically, in relation to the 9/11 attacks, the hijackers knew there was a martyrdom operation, but did not know about the nature of the operation until shortly before the attack for operational security reasons, the FBI memo states.

The documents were the latest materials to be released under an executive order from President Joe Biden aimed at making public long-classified investigative reports related to the attacks. A separate investigative document was released on the 20th anniversary of the attacks in September. The records have long been sought by victims’ relatives as they sue in federal court in New York to try to prove that the Saudi government was complicit, something Riyadh officials have vigorously denied.

The Saudi Embassy in Washington did not respond to a request for comment, but issued a statement in September calling any allegations of complicity malicious and categorically false.

US government investigations over the past two decades have documented support given by Saudi government officials to several of the hijackers upon their arrival in the US, but have not produced clear evidence that senior government leaders helped plot the attacks. The FBI memo closing out the investigation says the bureau has not identified additional groups or individuals responsible for the attack other than those currently charged.

Even so, the documents reveal new details about the years-long efforts by the FBI to hunt for possible involvement by the Saudi government and to scrutinise support given by Saudi nationals in the US to the first two hijackers to arrive in the US, Nawaf al-Hazmi and Khalid al-Mihdhar.

Andrew Maloney, a lead lawyer for the victims’ families, said the FBI has now released a substantial amount of very incriminating documents regarding the Saudi government’s role in helping Al Qaeda and these two hijackers in particular.

Brett Eagleson, whose father, Bruce, died in the World Trade Centre attack, said in a statement that the details in the documents help bolster the arguments that high-level Saudi officials aided and supported the 9/11 hijackers.

Among the episodes scrutinized by the FBI and recounted in the records is a February 2000 encounter at a Southern California halal restaurant between al-Hazmi and al-Mihdhar and a Saudi national named Omar al-Bayoumi, who subsequently helped them lease an apartment in San Diego. He had previously drawn FBI scrutiny but was never charged over his connections with the future hijackers.

According to the records released, the FBI also investigated ties between al-Hazmi and al-Mihdhar and people linked to the Saudi Ministry of Islamic Affairs, which funds mosques and efforts to promote Islam around the world. According to one of the documents, the FBI studied whether Al Qaeda operatives had infiltrated the ministry unbeknownst to the Saudi government or whether there was a collaboration of AQ operatives and certain radical elements within the Ministry of Islamic Affairs for mutually beneficial goals.