Relatives of people who died in the Twin Towers and the Pentagon during the 9/11 attacks picketed the White House on Tuesday demanding that President Barack Obama sign a bill that passed both chambers of Congress without opposition.

The White House says Obama intends to veto the bill, which would allow Americans to sue foreign countries including Saudi Arabia for alleged connection to terrorist attacks by greatly expanding a current exception to lawsuit-blocking sovereign immunity.

Obama has until Friday to make his decision final on the bill, and family members and supporters hoped to change his mind, though they chanted that they expect lawmakers to override the veto if one comes -- a potential first for the Obama presidency.

The Obama administration and some legal experts say the bill would create hazards for the U.S. government by upending the global status quo of reciprocal sovereign immunity, a widely accepted principle that countries generally can't be sued in foreign courts.

Americans since 1996 have been able to sue State Department-designated state sponsors of terrorism -- a short list of countries with poor relations with the U.S. The pending expansion would not require State Department designation.

Critics of the Justice Against Sponsors of Terrorism Act say the habitually rebel-funding, covert-war operating U.S. government has more at stake than countries that would naturally respond to the legislation by reducing U.S. protection under sovereign immunity.

Family members of 9/11 victims, however, say the act would serve as a powerful deterrent against terrorism and a tool to expose new facts in court.

The pro-JASTA rally at the White House was in just about every way unusual, and it was apparent that many participants were unpracticed in protesting.

Maureen Hunt wore a shirt commemorating her sister, Kathleen Hunt Casey, who died in New York. Hunt says she would have been working in the Twin Towers herself that day had she not taken off work as her daughter joined the military.

“It’s ridiculous our government can’t just pass it,” says Hunt, participating in her first protest. “Do we really need to do this?”

9/11 victim families chant 'no veto' at White House JASTA protest pic.twitter.com/hMCMDWpBiI — Steven Nelson (@stevennelson10) September 20, 2016

Mindy Kleinberg and Lorie Van Auken, whose husbands died in New York, marched up and down the sidewalk along Pennsylvania Avenue with signs that on one side had a New York Daily News cover telling Obama “Don't choose them over U.S.”

The other side showed a list of recent terrorist attacks inside the U.S., including the December murder of 14 people by a fanatical Pakistani-American couple in San Bernardino, California, the June murder of 49 people at a gay nightclub in Orlando by a religious Afghan-American and more recent nonlethal attacks attributed to a Somali-American in Minnesota and an Afghan-American in New Jersey and New York.

“One man is keeping us from making America safer,” Kleinberg says. “[The bill] would send a message to the world that if you come to our country and kill our citizens we’re going to hold you accountable.”

Kleinberg says the bill could discourage countries including Saudi Arabia from giving money to provide radical Islamic educations in madrassas in countries such as Pakistan, whose population arguably has been radicalized in part through such funding.

Van Auken says enacting JASTA doesn’t necessary mean Saudi Arabia would be found liable by a court, but that “we think we have a very strong case” and “we need to see the rest of the evidence.”

Many demonstrators walked a fine line, not definitively declaring the Saudi government responsible while asserting the bill would discourage and punish terrorism.

Alison Crowther, whose 24-year-old son Welles died in New York, says the bill would “allow us to choke off funding for terrorism.”

“I’ve never done this before,” Crowther said as she joined the picket line. Her son, nicknamed “the man in the red bandanna” by some 9/11 survivors, is credited with helping others escape the south tower of the World Trade Center.

Janet Carver and Sylvia Carver, whose sister Sharon died at the Pentagon, said they hoped Obama would sign the bill, noting it passed unanimously.

Janet Carver said the bill could have a deterrent effect, and Sylvia said she would view a veto as an injustice. Osama bin Laden has been held accountable, she says, and “now it’s time for the sponsors.”

Sylvia Carver, center, and Janet Carver, right, participate in a protest near the White House on Tuesday. Steven Nelson for USN&WR

“I wish I wasn’t here,” said Sean Passananti, whose father Horace died in the north tower.

“Unfortunately 15 years after 9/11 we have to fight for justice,” he said while holding a sign. “It’s a little mind boggling.”

Officials from Saudi Arabia, home to 15 of the 19 terrorists who hijacked airplanes on 9/11, have long been suspected of having at least some unexplained connection to the terrorists, whether or not they knew the men's intentions.

Twenty-eight long-withheld pages of initial congressional investigative work were mostly unredacted in July after a long campaign, showing some initial avenues of investigation into contacts between the terrorists and well-connected Saudis living in the U.S.

Columbia University law professor Lori Damrosch, an expert on sovereign immunity, says that in some cases it's already possible to sue a foreign country that is not a designated sponsor of terrorism. Chile, she says, was successfully sued after an agent in 1976 assassinated dissident Orlando Letelier with a car bomb in Washington, D.C.

The lawsuit against Chile was pursued under a narrow noncommercial tort exception to sovereign immunity.

"Basically all elements of the tort would have to happen in the U.S. Here it's a little more complicated," she says about 9/11. "If the Saudi ambassador carried a bomb with him into the State Department and blew up the State Department.... I would say that's the same as the Chilean car bombing."

But Damrosch says there apparently is no damning evidence on Saudi Arabia, or it would be declared a state sponsor of terrorism. She says JASTA, which would allow lawsuits to proceed against states that "knowingly or recklessly contribute material support or resources, directly or indirectly," could be broadly interpreted.

"That means a reckless Saudi paymaster of some sort who indirectly allowed funds to be transferred to someone who used them for terrible purposes would allow lawsuits to be brought against Saudi Arabia itself," Damrosch says.

University of Virginia law professor Paul Stephan says it's unlikely lawsuits would rake in any funds for victims, and says many countries other than Saudi Arabia could be targeted. A conspiracy theory holds that Israel actually organized the attacks, he points out.

Stephan opposes the legislation, saying it offers a poor method for retaliating against foreign-sponsored attacks and could disproportionately harm the U.S.

But, Stephan acknowledges, "some would argue the Saudis have bought up all of the U.S. government and we can't really trust the executive branch to vigorously investigate the Saudis, and that's why we need private lawsuits to get at the truth."