RNA - Mohammed's offer was disclosed late on Friday in a letter filed in the US District Court in Manhattan by lawyers representing individuals and businesses seeking billions of dollars in damages, The Wall Street Journal and Reuters news agency reported on Monday.
The Saudi government has long denied involvement in the 2001 attacks, in which hijacked airplanes crashed into New York's World Trade Center, the Pentagon outside Washington, DC, and a Pennsylvania field. Nearly 3,000 people died.
According to the letter, the plaintiffs' lawyers have been in contact with lawyers for five witnesses in federal custody about their availability for depositions.
The lawyers said three, including Mohammed, are housed at the Guantanamo Bay, Cuba detention camp, where they face capital charges, while two are at the "Supermax" maximum security prison in Florence, Colorado.
According to the letter, Mohammed would not agree "at the present time" to be deposed, but that could change.
"Counsel stated that 'the primary driver' of this decision is the 'capital nature of the prosecution' and that 'in the absence of a potential death sentence much broader cooperation would be possible'," the letter said, adding that Mohammed and the other Guantanamo detainees have been attending pre-trial hearings in their cases.
James Kreindler, a lawyer for the plaintiffs, told Reuters it was not clear how useful Mohammed might be, noting, "We're just really leaving no stone unturned."
Glenn Carle, a former officer with US Central Intelligence Agency, echoed a similar sentiment.
"He does know quite a bit about the structure of al-Qaeda, the individual decisions taken, how things happened. A lot of that was his thinking," Carle told Al-Jazeera, stating, "So, I think he does have information, certainly. Is it useable in a court of law in the United States is one of the big questions."
"The answer to that is not a clear yes, because information obtained has been tainted, the defence claims, and with cause, by having used illegal methods, enhanced interrogation, which is a euphemism for torture," Carle continued.
Al-Jazeera's Patty Culhane, reporting from Washington, DC, pointed out that the civil case of the 9/11 victims is separate from the criminal case Mohammed is facing.
She also stated that it is unclear if US President Donald Trump, who is close to the leaders of Saudi Arabia, would allow a plea deal for Mohammed to give evidence.
Bruce Fein, former US associate deputy attorney general, said the lawsuit had major financial implications for Saudi Arabia. However, the 2020 US presidential election could increase pressure on Trump to waive the death penalty for Mohammed.
"If the plaintiffs win in this case, it could be hundreds of billions of dollars. You have over 3,000 plaintiffs, compensatory plus punitive damages and a jury very hostile to Saudi Arabia, it could virtually bankrupt Saudi Arabia. All their assets in the US and elsewhere could be seized," Fein added.
"So, the incentive for Mr Trump as opposed to others to waive is not very great. But still, it may be in the year 2020 that the US population in general is not going to be sympathetic to Mr Trump running and seeking their vote if it looks like he's taking the side of Saudi Arabia over the victims of 9/11," Fein noted.
Saudi Arabia long had broad immunity from September 11 lawsuits in the US. But that changed in September 2016 when the US Congress overrode President Barack Obama's veto of the Justice Against Sponsors of Terrorism Act (JASTA).
In March 2018, US District Judge George Daniels in Manhattan, who oversees the litigation by victims, announced their claims "narrowly articulate a reasonable basis" for him to assert jurisdiction through JASTA over Saudi Arabia.
His decision covered claims by the families of those killed, roughly 25,000 people who suffered injuries and many businesses and insurers.
A previous attempt to broker a plea agreement with Mohammed and four other 9/11 defendants was scrapped over concerns that dropping the death penalty would serve as an official censure of the government's torture of the detainees.
A person familiar with the military proceedings told the Journal that one of the primary goals in those negotiations was gaining the defendants' cooperation.
"One of the main things that the 9/11 defendants have to offer is closure, particularly closure for the victims. With capital charges gone, there is an opportunity to tell the story of 9/11 once and for all," according to the person whom the Journal did not identify
In addition to the 9/11 attacks, the al-Qaeda member has claimed responsibility for the kidnapping and murder of American journalist Daniel Pearl in 2002.
Mohammed was captured in Pakistan in 2003 and has been held in the US's Guantanamo Bay prison since 2006.
The CIA subjected him to waterboarding 183 times in 2003, which former US President George W Bush later stated he personally authorised.