The float took the full force of the train at a railroad crossing Thursday afternoon. Some people managed to jump clear as the train, with its horn blasting, bore down on the float decorated with American flags. Horrified spectators at the parade could only watch as the carnage unfolded.

“The train honked its horn, but the 18-wheeler could not go anywhere because of the other one (truck) 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.”

Sudip Bose, a front-line physician in Iraq who had been volunteering at the parade, said Friday that the immediate aftermath of the collision reminded him of the war.

“It was a scene of total chaos,” Bose said, of Odessa, about 20 miles to the northeast.

He described how veterans were already tending to the wounded with limited medical supplies when he reached the crash site.

“Instincts kicked in. They were applying tourniquets, holding pressure to the wounds.” Bose said.

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

Of the 16 others hurt in the crash, four are in stable condition and one is critical. Ten others were treated and released from the hospital in Midland, about 320 miles west of Dallas.

The float was one of two flatbed tractor-trailers carrying veterans and their spouses. Police said the first truck safely crossed the tracks but that the second truck’s trailer was still on the crossing as the train approached.

Patricia Howle was waiting in her car at a nearby traffic light as the train approached.

“I just started screaming,” she said. “The truck was on the other side of the train, but I did see the panic on the faces of the people and saw some of them jump off.”

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.

“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.”

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.

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 canceled.

The events were organized by Show Of Support, a local veterans group. Its president, Terry Johnson, did not immediately return an email for comment and his phone number was unlisted; the phone rang unanswered at the group’s offices.

Secretary of Defense Leon Panetta “was deeply saddened by news of the tragic accident involving veterans heroes and their spouses in Midland,” Pentagon spokesman George Little said in a statement. “His thoughts and prayers are with the families of the victims, with those injured in this incident, and with the entire community.”

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Terry Wallace reported from Dallas. Associated Press writer James Beltran also contributed to this report from Dallas.