US president Barack Obama has vetoed a bill that would have allowed the families of 9/11 victims to sue the government of Saudi Arabia.
The move sets him up for a possible first veto override by Congress.
Both chambers passed the bill by voice vote.
The House sent President Obama the Bill just before the 15th anniversary of the attacks that killed nearly 3,000 people in New York, Washington and Pennsylvania on September 11, 2001.
Mr Obama says the Bill would "upset long-standing international principles regarding sovereign immunity".
The Bill would have given 9/11 families the right to sue in US court for any role that elements of the Saudi government may have played in the attacks.
Fifteen of the 19 men who carried out the attacks were Saudi nationals.