SCHOHARIE, N.Y. — They were young adults — sisters, brothers, old friends and newlyweds — who were celebrating one of their birthday's in a rented SUV limousine on a drive through rural Schoharie County.

They are now victims of the nation's deadliest transportation disaster in nine years.

Twenty people died Saturday afternoon in the horrific wreck at the intersection of state routes 30 and 30A in the town of Schoharie. The stretch limo, packed with 18 people, drove through a stop sign at the notorious intersection and fatally struck two people in the parking lot of a popular country store, before slamming into an earthen embankment and killing all of the oversized vehicle's occupants.

"This is the most deadly transportation accident in this country since February of 2009," Robert Sumwalt, chairman of the National Transportation Safety Board, told reporters at a police briefing on Sunday. He was referring to the Feb. 12, 2009, crash of Colgan Air Flight 3407 in Buffalo, which claimed the lives of 50 people.

"Twenty fatalities is just horrific," Sumwalt said of Saturday's deadly crash. "I've been on the board for 12 years, and this is one of the biggest losses of life that we've seen in a long, long time."

State Police would not release the names of the victims, saying they were still notifying family members Sunday afternoon. But a devastating picture began to emerge Sunday evening, as family members posted on social media and fundraising websites about the loss of four sisters from one family, as well as the deaths of their husbands and at least one of the husband's relatives.

Anthony Vertucci of Saratoga Springs confirmed to the Times Union that Amy Steenburg, her husband, Axel, his brother, Rich, and Amy's three sisters died in the accident. The group was celebrating a birthday. Vertucci said his niece, Erin McGowan, and her husband, Shane McGowan, were also in the limo and died in the crash. Relatives of the four sisters gathered at a house in Amsterdam Sunday night; two men outside the house declined to comment on the tragedy.

State Police set up a special hotline for the families of people who may have lost a relative in the crash. Family members are encouraged to call the State Police at 1-877-672-4911.

It marked the deadliest transportation incident in the Capital Region since the Oct. 2, 2005, sinking of the Ethan Allen tour boat on Lake George that killed 20 people.

Investigators from the NTSB are examining the road conditions at the time of the crash and whether any mechanical problems in the 2001 Ford Excursion limo may have been a factor. State Police First Deputy Superintendent Christopher Fiore provided little information Sunday on how the crash might have happened. Fiore would not identify the company that operated the limo.

Sumwalt estimated investigators will be at the scene for five days collecting evidence. He said investigators will not determine a cause of the crash until after they have completed their report.

"The fact that the NTSB is here indicates we're very concerned about this," Sumwalt said.

Officials said state law requires front-seat passengers in limos to wear seat belts, but not back-seat passengers. Police would not say if anyone in the limo was wearing a seat belt.

Schoharie County sheriff's deputies and troopers responded to the accident scene at 1:55 p.m. Saturday outside the Apple Barrel Country Store and Cafe.

The limo was traveling southwest and descending a steep hill on Route 30 when, according to State Police, it failed to stop at the intersection of Route 30A, where there is a stop sign. The limo traveled across the intersection into the Apple Barrel's parking lot and hit an unoccupied 2015 Toyota Highlander. Two pedestrians, who were standing nearby, were struck and killed.

Investigators cordoned off the area and had kept the cars of bystanders secured overnight Saturday while they investigated the crash scene and sought to reconstruct what happened.

Linda Riley, of Schenectady, returned to the store Sunday afternoon around 1 p.m. to retrieve her car.

Riley said she and her sisters had just pulled into the store's parking lot when the crash happened. Her car and other vehicles were covered with tree branches that splintered from the impact and debris from the collisions littered the ground.

They didn't realize a crash had occurred at first, she said.

"We thought it was an explosion," she said. "Someone said, 'There's a body laying right next to your car.' That was it. We just got out of the car and ran into the restaurant as quickly as possible."

Riley described a chaotic scene in a parking lot filled with customers.

"Everyone was screaming, 'Call 911, does anyone know CPR?' And then the cops arrived," she said. "I didn't want to come back to get my car. But I had to. You're in shock, just shock. You think about it all night."

The victims were all taken to Albany Medical Center Hospital, where autopsies will be done.

Gov. Andrew M. Cuomo issued a statement Sunday afternoon commending the emergency responders who worked through the night at the scene and had tried furiously to save lives.

"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," Cuomo said. "I join all New Yorkers in mourning these deaths and share in the unspeakable sorrow experienced by their families and loved ones during this extremely difficult time."

The Apple Barrel store sits at the bottom of a steep hill, where the speed limit is 50 mph. For those unfamiliar with the area, there is no indication a stop sign lies ahead on Route 30 until drivers are about 200 feet from the intersection.

Jessica Kirby, whose family owns the Apple Barrel store, said the intersection in front of the property is notoriously hazardous.

"This is a reoccurring issue in this intersection," she said.

Kirby said tractor-trailers have repeatedly come down the hill too fast and veered into the store's parking lot when customers are present. The state made improvements a few years ago, including eliminating truck traffic, but the intersection is still dangerous, she said.

"And if you were going too fast and you're not familiar with the area, you won't know it's a stop at the end of a steep hill," she said.

Kirby declined to discuss details of the carnage that she and others witnessed on Saturday.

"It's something no one should ever have to experience," she said. "Our thoughts and prayers are with the victims' families."

Schoharie town Supervisor Alan Tavenner confirmed the state Department of Transportation made improvements to the dangerous intersection about seven years ago, although concerns about the safety of the junction linger.

"There have been tractor-trailers that have come barreling down that hill and it was a miracle they didn't kill somebody," Tavenner said.

Staff writer Mallory Moench contributed to this story.