On Wednesday, the United States Congress voted overwhelmingly to override a veto by President Barack Obama, approving a new law that enables the relatives of those killed in the Sept 11, 2001, terrorist attacks to sue the government of Saudi Arabia in US courts.

The law could alter the US-Saudi relationship and cause economic instability that could unsettle the world's top oil exporter. The law would lift sovereign immunity and allow the Saudi leadership to be held responsible in US courts for the 9/11 attacks if victims' families can prove that any Saudi official played a role in the attacks.

US courts could seize the kingdom's assets in America to pay for any judgment. For their part, Saudi officials have threatened to sell off hundreds of billions of dollars in US holdings to avoid the reach of American plaintiffs. A rapid sale of assets could precipitate an economic crisis in Saudi Arabia, which could ripple to Asia and other parts of the world that are highly dependent on the kingdom's oil.

Fifteen of the 19 hijackers who carried out the 9/11 attacks were Saudis. For years, Saudi Arabia's leaders have argued that that fact is irrelevant - and that there is no evidence that Saudi officials or institutions provided a support network for Al-Qaeda and its hijackers. For a long time, Americans largely accepted that explanation.

Protesters participate in a rally to protest US President Barack Obama's plan to veto the Justice Against Sponsors of Terrorism Act (Jasta), in front of the White House. PHOTO: EPA

But in recent months, the facade of Saudi Arabia as America's most important ally in the Arab world and a force for stability in the Middle East began to crack. US public anger against Saudi Arabia is rising - over its war in Yemen, its treatment of women and dissidents, and the use of its oil wealth to export extremist ideology by building mosques and dispatching preachers throughout the Muslim world. Prodded by some relatives of the 9/11 victims, Americans want a re-examination of whether any Saudi official played a role in the attacks.

For more than a decade, US officials debated releasing a classified section of a 2002 congressional report on the attacks, which the George W. Bush administration ordered kept secret. The Obama administration finally declassified the material in July. But the release of the so-called "28 pages" did not end the debate over potential Saudi involvement.

In May, a former member of the 9/11 Commission, an independent bipartisan panel that investigated the attacks in 2004, told news organisations there was evidence that Saudi government employees helped some of the hijackers. "There was an awful lot of participation by Saudi individuals in supporting the hijackers, and some of those people worked in the Saudi government," said Mr John Lehman, a Republican who served as navy secretary in the Reagan administration and was among 10 commission members. "Our report should never have been read as an exoneration of Saudi Arabia."

The Saudis claimed vindication after the 9/11 Commission concluded it "found no evidence that the Saudi government as an institution or senior Saudi officials individually funded" Osama bin Laden or Al-Qaeda. (The report added it "does not exclude the likelihood that charities with significant Saudi government sponsorship diverted funds to Al-Qaeda".) The narrow wording left open the possibility that lower-level Saudi officials could have been involved in diverting funds to Al-Qaeda or supporting the hijackers. The two leaders of the 9/11 Commission - former New Jersey governor Tom Kean and former US congressman Lee Hamilton - have long argued that only one Saudi official, a former diplomat in the Saudi consulate in Los Angeles, was suspected of helping two Saudi hijackers in San Diego.

But Mr Lehman said the commission investigated at least five Saudi government officials, including employees of the kingdom's Ministry of Islamic Affairs, who potentially aided some of the hijackers. And former US senator Bob Graham, who was co-chairman of the 2002 congressional panel that investigated 9/11, insists that other Saudi individuals and institutions were complicit in the attacks.

Saudi Arabia has not faced such a sustained level of criticism from its American ally in decades. A group of US senators recently introduced a Bill to place new restrictions on US weapons sales to the kingdom because of its war in Yemen.

Of course, the US bears a significant part of the blame for its dysfunctional relationship with Saudi Arabia. Many Washington policymakers value the stability of the Saudi regime above all else, and for decades they have been willing to turn a blind eye to the ruling family's excesses and its support for Wahhabi extremism.

For years after 9/11, US officials tried to pressure their allies in Saudi Arabia and other "moderate" Arab regimes to crack down on financing for Islamic militants. But as State Department cables released by WikiLeaks revealed, the Saudis were reluctant to shut off the flow of cash - millions of dollars a year, often raised during the holy periods of Haj and Ramadan.

"It has been an ongoing challenge to persuade Saudi officials to treat terrorist financing emanating from Saudi Arabia as a strategic priority," then Secretary of State Hillary Clinton wrote in a December 2009 cable to US diplomats in the region. Eight years after 9/11, she noted that "donors in Saudi Arabia constitute the most significant source of funding to Sunni terrorist groups worldwide".

With the passage of the 9/11 families law, the US public and the world could get a fuller examination of Saudi Arabia's potential role in financing terrorism.

• Mohamad Bazzi is a journalism professor at New York University and a former Middle East bureau chief at Newsday.