Transcript for Limousine crash leaves 20 dead
We begin with the breaking news. The heartbreaking details of the celebration that turned to tragedy. A young woman's surprise birthday outing, ending with a limousine plowing into a parking lot, hitting a car before coming to a stop. 18 people were onboard, no one survived. Two pedestrians also killed. That death toll, the nation's worst accident on land or in the air since 2009. Tonight, the investigation is under way. As we're learning the first details about the victims. Erielle reshef is on the scene in New York. Reporter: Tonight, the NTSB investigating the country's deadliest transportation accident in nearly a decade. We need multiple ambulances. Med life on standby. Reporter: 20 people, killed on an upstate New York highway when officials say a limousine plowed through an intersection and into the parking lot of a restaurant, slamming into an unoccupied SUV. Investigators today sifting through the wreckage. This man carrying a wheel out of the bushes. From your experience, what's the magnitude of this? 20 fatalities is just horrific. This is the most deadly transportation accident in this country since February of 2009. Reporter: The incident happening around 2:00 P.M. Saturday. Be advised, we got two cars off in the brush. Reporter: According to the aunt of one victim, the limousine, a modified 2001 Ford expedition, was packed with people celebrating a birthday. All 18 inside including the driver, killed. I had four nieces. They're all dead. They were in there. Reporter: Two other pedestrians nearby struck in the crash did not survive. Witnesses describe a horrific scene. I heard this loud bang, then screaming. Reporter: Emergency crews from several areas dispatched to assist. This, the latest deadly accident involving a limo. In 2014, one person was killed and comedian Tracy Morgan was seriously injured. And in 2013, five people killed when this limo carrying a bachelorette party in San Francisco ignited. The passengers trapped inside. Erielle, this happened during the day. Did they have any idea what caused this? Reporter: Tom, investigators are looking into several factors, including whether alcohol or weather had an effect. It's still unclear if the people inside the limousine were wearing their seat belts. Tom? Erielle, thank you.
This transcript has been automatically generated and may not be 100% accurate.