Midland police also are investigating the crash, but it's not clear what, if any, criminal charges will be filed.
"I really can't speculate as to what any charges might be," said the county's district attorney, Teresa Clingman, because the police "investigation is not complete."
John Klassen, the U.S. attorney in Midland, said he's unaware of any federal criminal authorities investigating the accident.
According to the NTSB, the railroad crossing warning system was activated 20 seconds before the accident, and the guardrail began to come down seven seconds after that.
But some Midland residents have said there isn't enough time between when the signal begins and the trains arrive. They say guardrails aren't completely down by the time a train comes by.
The Union Pacific freight train heading to Louisiana was estimated to be going at 62 mph at the time of the crash.
The veterans had been invited to Midland, a transportation and commerce hub in the West Texas oilfields, for a three-day weekend of hunting and shopping in appreciation of their service. A local charity, Show of Support, had organized the trip, parade and other festivities. Show of Support officials have declined to identify the driver.
Killed were Marine Chief Warrant Officer 3 Gary Stouffer, 37; Army Sgt. Maj. Lawrence Boivin, 47; Army Sgt. Joshua Michael, 34; and Army Sgt. Maj. William Lubbers, 43.
According to Army records, Hayden holds the rank of staff sergeant, and his occupational specialty is light wheel vehicle mechanic.
He began his military career as an active-duty soldier from 1980 to 1988. He did two stints in the Oklahoma National Guard in the 1990s before signing up again for the Army Reserve in December 2003, according to Army records. He served in Iraq until March 2005 and was deployed to Afghanistan in 2009 until the following September.
His awards include an Afghanistan campaign medal with a campaign star, an Army commendation medal, an Army achievement medal and a combat action badge.