(Reuters) — Safety investigators descended on the West Texas oil town of Midland on Friday to search the wreckage of a freight train collision that killed four people on a parade float carrying U.S. military veterans and their spouses.

At least 16 people also were injured during the parade on Thursday afternoon, which was supposed to have kicked off a weekend of events, including a banquet and a hunting expedition, to honor wounded veterans.

The National Safety Transportation Board said it was looking into the crash and had sent investigators to the scene.

"Our team is in route," NTSB Chairman Debbie Hersman said on NBC's "Today" show on Friday. "We want to make sure that we can identify what happened and why it happened so we can prevent things like this from happening in the future."

Two of the victims died at the scene and two at a Midland hospital, police have said.

One of those hospitalized was in critical condition on Friday, a hospital spokesman said. Another was airlifted to a medical facility in Lubbock, Texas, on Thursday, police said. Four others were in stable condition and 10 were treated and released, according to police.

The tragedy happened as two flatbed trailers carrying veterans from the Iraq and Afghanistan wars, some who suffered major injuries in combat, attempted to cross railroad tracks during the "Hunt for Heroes" parade.

"The first flatbed crossed the train tracks completely. The second did not make it across before being struck by the train," a police statement said.

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Veterans and their spouses tried to jump off the trailers to escape the collision. There were 26 people on the float that was hit by the Union Pacific Corp. train, including a dozen veterans, a dozen spouses and two escorts.

"It's hard to look at. It's a very tragic event, very unfortunate," Midland Police Chief Price Robinson said from the site of the accident on Thursday.

A Union Pacific spokesman referred all questions to the NTSB.

Many of the 25 West Texas veterans being honored served multiple deployments in Iraq and Afghanistan, according to biographies posted on a website created by event organizers. They were described as having been shot on the battlefield or wounded by improvised explosive devices.

Some described suffering traumatic brain injuries and post-traumatic stress disorder as a result of their deployments, the biographies said.

The Pentagon said in a statement that Defense Secretary Leon Panetta, who is traveling in Asia, "was deeply saddened by news of the tragic accident."

Authorities did not immediately release the names of those injured or killed.

A Facebook page for Show of Support, which organized the "Hunt for Heroes" event, was filled with condolences on Friday morning.

"My thoughts & prayers go out to my fellow Veterans & their families & the whole community in Midland. This is such a tragic accident. So sad," said one message, posted by Jeff A. Bingham.

Others raised questions about the dangers posed by the parade route. "Taking them across those tracks in that manner and by NOT ENSURING their safety by ensuring traffic would NOT STOP was unnecessarily placing them in harms way," posted Mitchell R. Mace.