TRUXTON, N.Y. -- Even seasoned firefighters said they couldn't believe what they saw when they responded to a crash that killed seven people in upstate New York:

A trailer hauling crushed cars broke free of the big rig that was pulling it, slashed across a rural, two-lane road and slammed into a mini-van whose driver tried desperately to avoid the runaway rig.

Four children, ages 4 through 7, and three adults died in the crash Wednesday night in Truxton, about 25 miles south of Syracuse. Another adult was hospitalized in good condition. Police are investigating the cause of the crash.

Ken Stone, first assistant chief of the Truxton Fire Department, said responders were deeply affected by a crash that seemed so improbable.

"My god! What did we just go through?" he said of the firefighters' reaction. "What did we just see? Did that just really happen?"

"You get all that training and you're not ready for it," he said. "You can't be if you're human."

Stone was one of about 20 firefighters who responded. He said it appears the van's driver saw the trailer coming and drove off the road trying to avoid it.

"They tried to miss it, but he ran out of time," said Stone. "All of a sudden, a trailer is coming at you and there's no truck on it.

"Tore it right apart," he said. "It was humbling, especially young kids. It tears you up."

The accident Wednesday night claimed the lives of 26-year-old Teresa Bush and her daughters, 4-year-old Alexis and Jasmine, 5. Also killed were 21-year-old Lena Beckwith; Carino Vanorden, 24; Alyssa Mead, 7; and Tyler Mead, 4.

Shawn Mead, the father of the two Mead children, was transported to Upstate University Hospital in Syracuse where he was in good condition.

All of the victims were from the local area.

In a posting on a Facebook page that apparently belonged to Beckwith, she said she and Vanorden had been engaged since 2011.

Officials said the trailer disconnected from the southbound rig and slammed into the northbound van, ripping it apart. Both vehicles came to rest on the shoulder of the two-lane road.

Stone said a couple of victims were under the trailer, which had to be jacked up to get them out. He said they all likely died upon impact.

"It was all instant, no doubt," he said.

Family members gathered Thursday morning at a farmhouse about a mile from the accident site. A woman who met reporters asked for privacy and said the family would talk when they are ready.

A person who answered the phone at Vanorden's declined to comment.

Sheriff's Capt. Mark Helms said authorities are looking at everything from mechanical failure to driver actions.

The Newton Salvage truck was driven by Ryan Dorward, 26, of Manlius, who was accompanied by Duane Newton. Messages left with Newton and the Georgetown, N.Y.-based salvage company were not returned Thursday afternoon. Dorward's telephone mailbox was full and could not accept messages.

Neither man was injured.

In New York, trucks are required to be inspected at least once a year and are subjected to random roadside checks by the Department of Transportation or state troopers, said Jennifer Post, spokeswoman for the DOT. Drivers are also required to do a "walk-around" inspection of their trucks every time they make a trip, checking -- among other things -- tire pressure and condition, brakes and the coupling between truck and trailer.

The salvage company has been in business since last year and had one roadside inspection by troopers in March of this year, according to the DOT. The inspection found minor violations on the truck, including being 4 per cent overweight, having no reflective tape and no fire extinguisher. None of them resulted in the truck being taken out of service.

Truxton is a town of 1,100 located a few miles northeast of the city of Cortland in the centre of New York.

"It's the worst thing I've ever seen. It's just a horrible thing. It gives me goose bumps," said Ann Maxson, 74, a lifelong Truxton resident.

A sign along Route 13 near the town line reads: "Please drive safely. We (heart) our children."