By Matthew Waller

MIDLAND, Texas (Reuters) - All four men killed when a freight train collided with a parade float honoring wounded American war veterans had served in the U.S. Army, police said on Friday, as investigators descended on the West Texas oil town of Midland to search the wreckage.

At least 16 other people were injured during the parade on Thursday at the start of what was meant to be a weekend of events including a banquet and a hunting expedition to salute U.S. veterans of the Iraq and Afghanistan wars.

Midland police identified the dead as: Sergeant Major Gary Stouffer, 37; Sergeant Major Lawrence Boivin, 47; Sergeant Major William Lubbers, 43; and Sergeant Joshua Michael, 34.

In Washington, the Army said Lubbers was retired from the military but could not immediately confirm whether the others were retired or active duty at the time of the accident.

The National Transportation Safety Board (NTSB) sent 16 experts to the scene to examine evidence including video from a forward-facing recorder on the train, officials said.

One person remained in critical condition and four were stable on Friday at a Midland hospital, a spokesman said. Another person was in serious but stable condition at a Lubbock hospital. Ten more were treated and released, police said.

The tragedy occurred as two flatbed trailers carrying the veterans, some of whom suffered major injuries in combat in Iraq and Afghanistan, 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.

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

"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.

The train and trailer remained in place on Friday morning at the site of the crash.

The NTSB said it would not determine a probable cause of the accident or provide any analysis while its investigators were at the scene. It said it had not yet determined if the grade-crossing warning system was working at the time of the accident.

A Union Pacific Corp spokesman referred questions on the investigation to the NTSB. Continued...