In a file photo Khalid Sheikh Mohammed, the alleged Sept. 11 mastermind, is seen shortly after his capture during a raid in Pakistan Saturday March 1, 2003, in this photo obtained by the Associated Press.

Khalid Sheikh Mohammed, the alleged mastermind of the Sept. 11 terror attacks, might help the families of victims of those attacks with testimony in their civil lawsuit against the government of Saudi Arabia if the U.S. government abandons a bid to execute him via military tribunal, a new court filing revealed.

Lawyers for the victims' families, in a letter to the judge overseeing the case, said they had learned from Mohammed's lawyers that he is not willing to be deposed in that civil case "at the present time."

KSM's attorneys "stated that the 'primary driver' of this decision is the 'capital nature of the prosecution" against the former top lieutenant to the late al-Qaeda leader Osama bin Laden, according to the letter from the plaintiffs' lawyers, whose clients allege that the Saudi government supported and funded the 9/11 attacks.

"In the absence of a potential death sentence much broader cooperation would be possible," KSM's attorneys told the plaintiffs' lawyers, according to their letter filed Friday with Judge Sarah Netburn in U.S. District Court in Manhattan.

The letter was first reported by The Wall Street Journal on Monday, the same day President Donald Trump signed into law a bill to replenish billions of dollars in funds for the victims of the 9/11 attacks.

The 55-year-old KSM has been held for nearly 13 years at the U.S. military base at Guantanamo, Cuba, where he and four other accused terrorists are charged with war crimes related to the 9/11 attacks using three hijacked airplanes on the World Trade Center in New York City and the Pentagon, along with the fourth hijacking of a commercial airliner that crashed near Shanksville, Pennsylvania, after first being flown toward Washington, D.C. Fifteen of the 19 hijackers were Saudi nationals.

Nearly 3,000 people perished in those attacks.