By Holly Yan, Mark Morales and Polo Sandoval, CNN

(CNN) -- [Breaking news update, posted at 12:43 p.m. ET]

The driver of the modified limo that crashed in upstate New York "did not have the appropriate driver's license to be operating that vehicle," Gov. Andrew Cuomo said Monday.

He also said the vehicle -- which was an SUV modified into a limousine -- "was inspected by the New York State Department of Transportation last month and failed inspection and was not supposed to be on the road."

Twenty people died in Saturday's crash in Schoharie, including the limo's 17 passengers, the driver and two pedestrians.

[Previous story, published at 11:02 a.m. ET]

This was a limo filled with exuberance -- 17 birthday party guests who had many reasons to celebrate.

There were newlyweds and young couples and four sisters, all on their way to revel at an upstate New York brewery.

But for reasons still unknown, the modified limo plowed through a stop sign, crashed into a parked SUV and caused the deadliest US transportation accident in almost a decade.

All 17 passengers were killed. So was the limo's driver. So were two pedestrians in the quiet town of Schoharie.

As their families grapple with confusion and grief, investigators are wondering whether the unusual structure of the limo may have contributed to the mass tragedy.

One family loses four sisters

Those in the limo weren't just friends -- many were family.

Four sisters -- Mary Dyson, Abby Jackson, Allison King and Amy Steenburg -- all perished in the crash, state Assemblyman Angelo Santabarbara said. Steenburg's husband Axel Steenburg also was killed.

Many of the victims were from the upstate city of Amsterdam, about 20 miles north of the crash site. Jackson was a special education teacher in Amsterdam, said Santabarbara, who represents the part of New York where the crash happened.

Valerie Abeling said her niece, Erin Vertucci and Erin's husband Shane McGowan, died together. They got married just four months ago.

"It's a horrible tragedy, and there's no words to describe how we feel," Abeling said.

"These were young couples, just got married and had their whole lives ahead of them."

Karina Halse said she's struggling with the loss of her sister, Amanda Halse, who was killed along with her boyfriend.

"My heart is completely sunken," Karina said. "I can't even imagine how it happened, or why it happened."

And Barbara Douglas isn't just grieving the deaths of her two nieces. She's mourning the loss of two mothers.

"They were fun-loving. They were wonderful girls," Douglas said. "They'd do anything for you, and they were very close to each other."

Douglas' face grew increasingly somber as she thought of her nieces' three children.

"They now have no parents," Douglas said.

Questions abound over the structure of the limo

Federal, state and local investigators flooded the tiny town of Schoharie to try to understand what happened.

The crash happened outside an Apple Barrel Country Store & Cafe. Resident Bridey Finnagen said it was loud enough to hear from down the road.

"I heard a loud bang. I came out my front door to see what was going on," Finnagen told CNN affiliate WTEN.

"I saw a lot of people here at the Apple Barrel out in the parking lot. Then I heard screaming. Then I saw this large van, a very unusual looking vehicle, out here in Schoharie in the bushes and really wrecked, hit a tree."

The vehicle appeared to be a 2001 Ford Excursion SUV modified into a limo, officials said.

These kinds of altered vehicles have worried officials, said Peter Goelz, former managing director of the National Transportation Safety Board. That's because after-market modifications often affect a vehicle's structural integrity and safety.

It's not clear whether the driver was speeding, whether the brakes were working, or whether the passengers were wearing seat belts, said Chris Fiore of the New York State Police. In these kinds of limousines, rear passengers are not required to wear seat belts, Goelz said.

The crash was so catastrophic, it stunned even veteran experts, such as the chair of the NTSB.

"Twenty fatalities, it's just horrific," board chairman Robert Sumwalt said.

"I've been on the board for 12 years and this is one of the biggest losses of life ... This is the most deadly transportation accident in this country since February of 2009."