Now, 15 years later -- and two days after Congress legally paved the way -- she's filed a lawsuit against Saudi Arabia, claiming the kingdom is partially responsible for his death.
In court documents filed Friday in Washington, D.C., DeSimone alleges Saudi Arabia provided material support to al Qaeda for more than a decade and was aware of the terror group's plan to attack the US.
"Absent the support provided by the Kingdom, al Qaeda would not have possessed the capacity to conceive, plan, and execute the September 11th attacks," the documents say.
The lawsuit alleges that the plaintiffs -- which include DeSimone's daughter -- suffered "severe and permanent personal injuries" and are seeking unspecified compensation.
The documents go on to allege Saudi Arabia, through agents and purported charities, provided al Qaeda members with financial and other logistical support to carry out the attacks.
Messages left Saturday with the Saudi embassy in Washington were not immediately returned.
The lawsuit comes just days after Congress voted to override President Barack Obama's veto of the Justice Against Sponsors of Terrorism Act, which allows Americans to sue a foreign state for acts of terrorism.
Congress voted overwhelmingly to override the veto -- the first time it has done so during Obama's presidency -- but the President warned it could damage the America's relationship with Saudi Arabia and even open the US government up to lawsuits by other foreign states for acts by the military.
One day after Congress passed the bill, top congressional leaders from each party expressed buyer's remorse
over the legislation, calling for changes to be made to the law. House Speaker Paul Ryan specifically called for a "fix" to protect service members overseas.
Earlier this year, the US released portions of a formerly classified Congressional report showing some of the 9/11 hijackers were in contact with and received support from individuals likely connected to the Saudi government.
Fifteen of the 19 9/11 attackers were Saudi citizens.
The Saudi Foreign Ministry put out a statement earlier this week saying, "The enactment of JASTA is of great concern to the community of nations that object to the erosion of the principle of sovereign immunity, which has governed international relations for hundreds of years."
"The erosion of sovereign immunity will have a negative impact on all nations, including the United States."