The US Senate has passed legislation that would allow families of 9/11 victims to sue the government of Saudi Arabia.
The measure, which must still be voted on by the House of Representatives, was approved despite fierce objections from the US ally and the Obama administration.
Of the 19 hijackers, 15 were from Saudi Arabia, with the others from the United Arab Emirates, Egypt and Lebanon.
Riyadh, which denies responsibility for the 2001 attacks, has reportedly threatened to sell up to $750bn in US assets if the bill becomes law.
The Justice Against Sponsors of Terrorism Act gives victims' families the right to sue in US court for any role that elements of the Saudi government could have played in 9/11.
Relatives have been calling on the Obama administration to declassify 28 pages of a US intelligence report, which are said to discuss possible Saudi involvement.
The bill would remove sovereign immunity - which shields governments from lawsuits - for countries found to be involved in terrorist attacks on US soil.
Saudi Foreign Minister Adel bin Ahmed Al-Jubeir said earlier this month such a move would "turn the world for international law into the law of the jungle".
New York Democrat Chuck Schumer, co-sponsor of the bipartisan bill, said: "Today the Senate has spoken loudly and unanimously that the families of victims of terrorist attacks should be able to hold the perpetrators, even if it's a country, a nation, accountable."
Senate Democrats firmly supported the legislation, putting them at odds with the Obama administration, which has threatened a veto.
The White House has said the bill could expose Americans overseas to legal risks.
"Given the concerns that we've expressed, it's difficult to imagine the president signing this legislation," White House spokesman Josh Earnest told reporters.
But Senator Schumer said the Senate had the two-thirds vote the chamber needs to override a presidential veto.
