SCHOHARIE, N.Y. — A limousine on its way to a birthday party blew a stop sign at the end of a highway, killing all 18 people in the limo and two pedestrians in the deadliest U.S. transportation accident in almost a decade, officials and a relative of one of the victims said Sunday.

The collision turned a relaxed Saturday afternoon into chaos at an upstate New York spot popular with tourists taking in the fall foliage. An aunt of one of the victims in the vehicle said the group had been traveling to a birthday celebration.

The 2001 Ford Excursion limousine was traveling southwest on Route 30 in Schoharie, about 170 miles (270 kilometers) north of New York City around 2 p.m. when it failed to stop at a T-junction with state Route 30A, State Police First Deputy Superintendent Christopher Fiore said at a news conference in Latham, New York.

It went across the road and hit an unoccupied SUV parked at the Apple Barrel Country Store, killing the driver and 17 passengers, as well as two people outside the vehicle.

The crash "sounded like an explosion," said Linda Riley, of nearby Schenectady, who was on a shopping trip with her sisters and had been in their parked car at the time at the store.

When she got out of her vehicle, she saw a body on the ground, she said. People started screaming.

The store manager, Jessica Kirby, told The New York Times that the limo was coming down a hill at "probably over 60 mph."

Authorities said autopsies were being performed and didn't release names of victims or speculate on what caused the limo to run the stop sign. The National Transportation Safety Board is investigating.

Speaking through tears, Valerie Abeling, said her 34-year-old niece Erin Vertucci was among the victims, along with her newlywed husband, 30-year-old Shane McGowan. They were on their way to a friend's birthday party, she said; her own daughter had been invited along but couldn't go.

"She was a beautiful, sweet soul; he was too," Abeling said.

The couple was married at a "beautiful wedding" in June at a venue in upstate New York, Abeling said. "They had everything going for them."

Vertucci, who grew up in Amsterdam, New York, was an administrative assistant at St. Mary's Healthcare in Amsterdam, Abeling said.

At the news conference, Fiore didn't comment on speed, or whether the occupants of the vehicle had been wearing seat belts.

"This is one of the biggest losses of life that we've seen in a long, long time," NTSB Chairman Robert Sumwalt said at the news conference.

It's the deadliest transportation accident since February 2009, when Colgan Air Flight 3407 crashed in Buffalo, New York, killing 50 people, Sumwalt said.

And it appears to be the deadliest land-vehicle accident since a bus ferrying nursing home patients away from Hurricane Rita caught fire in Texas 2005, killing 23.

There was no information Sunday on the limousine, its origin or its integrity. But safety issues on such vehicles have arisen before, most notably after a wreck on Long Island in July 2015 in which four women on a winery tour were killed.

They were in a Lincoln Town Car that had been cut apart and rebuilt in a stretch configuration to accommodate more passengers. The limousine was trying to make a U-turn and was struck by a pickup.

A grand jury found that vehicles converted into stretch limousines often don't have safety measures including side-impact air bags, reinforced rollover protection bars and accessible emergency exits. That grand jury called on New York Gov. Andrew Cuomo to assemble a task force on limousine safety.

Limousines built in factories are already required to meet stringent safety regulations, but when cars are converted into limos, safety features are sometimes removed, leading to gaps in safety protocols, the grand jury wrote.

On Sunday, Cuomo released a statement saying, "My heart breaks for the 20 people who lost their lives in this horrific accident on Saturday in Schoharie. I commend the first responders who arrived on the scene and worked through the night to help. State police are working with federal and local authorities to investigate the crash, and I have directed state agencies to provide every resource necessary to aid in this investigation and determine what led to this tragedy."

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Salsberg reported from Boston. Associated Press writers John Kekis in Latham, New York, and David Klepper in Providence, Rhode Island, contributed to this report.