U.S. officials are heading to West Texas to join investigators looking into why a freight train slammed into a parade float carrying wounded veterans.

A joyful celebration turned to chaos as a train, its horn blaring, barreled into a float laden with military veterans and their spouses on their way to a banquet to honor the returning war heroes.

At least four veterans were killed in the collision Thursday afternoon, with another 17 others injured.

The float took the full force of the train at a railroad crossing in Midland. Some managed to jump clear as the train bore down on the float decorated with American flags. Horrified spectators could only watch as the carnage unfolded.

Pam Shoemaker from Monroe, La., said she and her husband, a special operations veteran, were on the float ahead of the one that was struck. Shoemaker described how the celebration so quickly turned sour.

She said her truck had just crossed the tracks and was moving slowly but never stopped. All around, the crowds lining the parade route cheered.

"It was beautiful," she said Friday. "There were lots of people with signs. Children yelling `thank you!' waving flags."

Then they heard the train coming. There was no warning -- she hadn't seen or heard it until it was upon them. The Shoemakers jumped from their truck and ran toward the other one, knowing it would be hit in a matter of seconds. The crossing barriers had just started to come down, she said.

"We started to jump off of our trailer. We saw people jumping from the other trailer and then there was the impact," Shoemaker said.

Sudip Bose, who was a front-line physician in Iraq, said Friday that the immediate aftermath reminded him of a combat triage situation. Veterans were already tending to the wounded when he reached the crash site. Bystanders tried to help with the limited medical supplies available.

"Instincts kicked in. They were applying tourniquets, holding pressure to the wounds," said Bose, who served in Fallujah and Baghdad and was volunteering at the parade.

Sixteen other people were hurt in the crash.

'Trained for tragedy'

"It was a scene of total chaos," said Bose, of nearby Odessa.

Shoemaker credited the training and courage of the veterans who jumped to help the injured. Her husband, Tommy, resuscitated one person and applied a tourniquet to a bleeding woman.

'It was a horrible accident to watch happen right in front of me.'—Witness Daniel Quinonez

"They are trained for tragedy," Shoemaker said.

Spectators described their horror and helplessness as the train hurtled toward the flatbed tractor-trailer that was unable to move because of other floats in the parade.

"The train honked its horn, but the 18-wheeler could not go anywhere because of the other one being right in front of it," said Daniel Quinonez, who was waiting in his vehicle as the parade went by. "It was a horrible accident to watch happen right in front of me. I just saw the people on the semi-truck's trailer panic, and many started to jump off the trailer. But it was too late for many of them."

Sgt. Maj. Gary Stouffer, 37, and 47-year-old Sgt. Maj. Lawrence Boivin were pronounced dead at the scene, Midland police spokesman Ryan Stout said. Army Sgt. Joshua Michael, 34, and 43-year-old Sgt. Maj. William Lubbers died later at Midland Memorial Hospital.

Of those hurt, four are in stable condition and one is critical, Stout said. Ten others were treated and released from the hospital in Midland, about 320 miles west of Dallas.

At a prayer vigil Friday morning, Mayor Wes Parry's voice cracked as he described how he had met Boivin and his wife a day earlier.

"It's hard to believe today that he's not here anymore," Parry said.

Deborah Hersman, NTSB chairwoman, said Friday on NBC's Today show that the train was equipped with a forward-facing camera whose footage could help in the investigation. NTSB officials were heading to Midland on Friday to investigate the crash.

"That will give us some video images if it survived the crash and we can download it, as well as recorders on the train," Hersman said. "We're going to be looking at the signals ... and making sure that the gates and lights were coming down.

10 previous accidents

Late Thursday, Union Pacific spokesman Tom Lange said a preliminary investigation indicated the crossing gate and lights were working. He did not know if the train crew saw the float. The black box from the train will determine its speed at the time of impact.

Federal Railroad Administration records reviewed by The Associated Press show that there have been 10 previous collisions — five cars and five trucks — at the same railroad crossing since 1979. Six drivers were injured in those accidents, but there were no fatalities. The trains involved were moving slowly at the time of the previous accidents, between 15 and 25 miles per hour.

The parade had been scheduled to end at a "Hunt for Heroes" banquet honoring the veterans. The wounded service members were then going to be treated to a deer-hunting trip at the weekend. The events were cancelled.

The events were organized by Show Of Support, a local veterans group that says its mission is to "lift the spirits of our U.S. troops and disabled veterans" through hunting and fishing. The group's president, Terry Johnson, has not responded to emails seeking comment and his phone number was unlisted; the phone rang unanswered at the group's offices.