Some of the 9/11 hijackers, who flew passenger planes into the New York World Trade Centre in 2001, "were in contact with and received support or assistance from individuals" who may have been connected with the Saudi Arabian government, according to newly declassified US report into the attacks.

Known as the "28 pages", the document, which was part of a 2002 Congressional Joint Inquiry into the attacks, has remained secret since its completion despite repeated calls for it to be made public.

It was produced shortly after 19 hijackers flew two planes into the Twin Towers at New York's World Trade Center and a third into Pentagon. A fourth plane, United Airlines Flight 93 was steered toward the US capital Washington, DC, but crashed into a field near Shanksville, Pennsylvania, after its passengers tried to overcome the hijackers.

The attacks killed 2,996 people and injured more than 6,000, causing at least $10bn (£7.8bn) in property and infrastructure damage.

"While in the United States, some of the September 11 hijackers were in contact with, and received support or assistance from, individuals who may be connected to the Saudi Government," the report said. Fifteen of the 19 hijackers were Saudi citizens.

Information, primarily from FBI sources indicated that "at least two of the individuals were alleged by some to be Saudi intelligence officers", the report said. One of them may have received a fake passport from his government's officials and a "significant amount" of money from one its royals, it added.

Saudi officials are also alleged to have provided "substantial assistance" to hijackers Khalid al-Mihdhar and Nawaf al-Hazmi.

A reported Saudi interior ministry official who stayed at the same Virginia hotel as Hazmi in September 2001 denied knowing the hijackers. But the FBI "believed he was being deceptive", the inquiry said. During questioning, he was suspected of faking a seizure during questioning and was able to leave the US, although the FBI attempted to interview him again.

The Joint Inquiry also confirmed that the US intelligence community had information that "individuals associated with the Saudi Government in the United States" may have had ties with al-Qaeda.

Intelligence also suggested Osama bin Laden's half-brother worked at the Saudi embassy in Washington DC, it added.

However, the report, which actually contains 29 pages of material and a letter from then-CIA Director George Tenet, also admitted that much of that information had "yet to be independently verified".

It also criticised the lack of effective intelligence sharing between various US government agencies. The report highlighted a CIA memorandum "which discusses alleged financial connections between the September 11 hijackers, Saudi Government officials, and members of the Saudi Royal Family" that was placed into an FBI case file, but not forwarded to FBI headquarters until it was discovered by the inquiry.

"Prior to September 11th, the FBI apparently did not focus investigative resources on [redacted] Saudi nationals in the United States due to Saudi Arabia's status as an American 'ally'," it said. However, it said that a number of FBI agent and CIA officers had complained to the inquiry "about a lack of Saudi cooperation in terrorism investigations both before and after the September 11 attacks".

One New York-based FBI agent told them that "the Saudis have been useless and obstructionist for years". The report said another FBI agent had provided copies of Saudi national passports to their government after 9/11, but received no information about them.

Victims' families and politicians have been calling for the report's release for years. President Obama said in April that his administration would allow it to be made public, although small sections remained redacted.

Its release was welcomed by New York Senator Charles Schumer, who told reporters that "if the Saudi government was complicit in 9/11, they should pay the price to the families who deserve justice".