Witnesses described people screaming as the warning bells at the crossing went off and the train blasted its horn.
Daniel Quinonez, who was waiting in his vehicle as the parade went by, said the float on the tracks could not go anywhere because of the one right in front of it.
"It was a horrible accident to watch happen right in front of me," he said. "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."
Another witness, Joe Cobarobio, said only a few seconds elapsed between the time the crossing gates came down and the train slammed into the flatbed truck with a "giant cracking sound."
Michael, one of the soldiers killed, pushed his wife off the float when he saw the train coming, his wife told Cory Rogers, a friend of the couple.
"His first instinct was to get her out of harm's way," said Rogers, who was not at the parade. "That's the kind of man he was, and I feel like it was his training as a paramedic and then as a soldier, choosing to put someone's life before your own."
Federal Railroad Administration records reviewed by The Associated Press show there were 10 collisions at the crossing between 1979 and 1997. But no accidents had happened in the past 15 years, the NTSB's Rosekind said.
Six drivers were injured in those accidents. The trains involved were moving slowly at the time, between 15 and 25 mph.
A key question for investigators is whether, after the speed limit was raised, the timing of the crossing gates was changed to give cars and trucks enough time to clear the tracks, Robert Chipkevich, who headed NTSB's rail investigations unit until retiring in 2010, said in an interview.
Investigators will also look at whether traffic lights in town prevented the flatbed truck in front from moving ahead, he said.
Sudip Bose, who was a front-line physician in Iraq, said the aftermath reminded him of a combat triage situation. Veterans instantly tended to the injured, and bystanders helped, too. Shoemaker's husband, Tommy, resuscitated one person and applied a tourniquet to a bleeding woman.
"Instincts kicked in," said Bose, who served in Fallujah and Baghdad and was volunteering at the parade.
Associated Press writers James Beltran, Nomaan Merchant, Danny Robbins and Terry Wallace in Dallas; Angela K. Brown in Fort Worth, Texas; and Joan Lowy in Washington contributed to this report.