US President Joe Biden on Friday ordered that the still-classified documents from the US government's investigation into the September 11 attacks be released within the next six months.

Biden's approval comes in response to pressure from the families of the nearly 3,000 people who were killed by al-Qaeda on September 11, 2001.

The families have long considered that the classified documents may contain evidence that the government of Saudi Arabia, a close ally of Washington, had links to the hijackers who attacked the World Trade Center and the Pentagon with civilian aircraft.

"Today, I signed an executive order directing the Department of Justice and other related agencies to oversee a review of the declassification of documents related to the FBI's 9/11 investigation," Biden said.

He added that the lifting of confidentiality should come into effect in the "next six months."

"We must never forget the continuing pain of the families and loved ones of the 2,977 innocent people killed during the worst terrorist attack against America in our history," Biden said.

This move comes before the twentieth anniversary of the attack, which prompted then-President George W. Bush, shortly after, to issue orders to invade Afghanistan, when the Taliban were providing a haven for al-Qaeda leaders.

Biden withdrew the last US soldier from Afghanistan this week, ending an unprecedented evacuation from Kabul airport, after the Taliban seized power again.

The pressure for more information is prompted by families of the victims who are suing Saudi Arabia for alleged complicity in the attacks. Families have long expressed frustration with the number of documents that remain classified and not viewed.

The official 9/11 commission set up by Congress said there was "no evidence that the Saudi government as an institution or senior Saudi officials unilaterally provided funding" to al-Qaeda.

Some took this wording as an indication that unofficial or lower-ranking Saudi figures may have played a role in the attacks.

Part of the investigation remains classified as highly classified and cannot be published.

In the executive order that launched the declassification process, Biden noted that "the events occurred two decades or more ago, and they relate to a tragic moment that still recurs in American history."

"Therefore, it is important to ensure that the US government maximizes transparency (...)".