(ABC News) -- The warning signals at a railroad crossing were activated seven seconds before a flatbed truck full of wounded veterans crossed the tracks during a parade and was struck by a freight train, according to preliminary details released by the National Transportation Safety Board.
Four veterans were killed and 16 were injured in the accident, which happened on Thursday.
It is not yet clear when the train conductor applied the brake, but the question is one of dozens the board hopes to answer in the coming weeks.
"Our mission is to determine probable cause, which is to determine not just what happened but why," NTSB Board Member Mark Rosekind said. "And that why is critical for us to determine what safety recommendations need to be issued so this does not happen again."
Investigators will be on scene in Midland for seven to 10 days collecting evidence to bring to Washington, D.C., for processing. They will examine both vehicles and the gates that should have blocked the tracks. They will also contact all drivers, victims and witnesses.
Given that the area where the crash happened is a "quiet zone," Rosekind said his team will be looking into whether that played a role in the collision.
Established for train crossings near residential areas, quiet zones prohibit train engineers from sounding their horns unless there is an emergency, according to the San Antonio Express-News. These zones are also equipped with extra safety equipment, and it's up to the engineer to determine whether a situation warrants the horn.
Witnesses have said they heard the train sound its warning whistle just before the crack of the train hitting the flatbed full of wounded war veterans and their wives on their way to a banquet.
Trained for emergencies, many veterans jumped from their truck seconds before the train hit and immediately returned to help the wounded.
Sudip Bose, a front-line physician in Iraq, said his "instincts kicked in." He and many others, including special operations soldier Tommy Shoemaker, were quick to make tourniquets and resuscitate fellow veterans and their spouses.
At least two of the four men killed gave their lives to push their wives from danger, witnesses said.
Army Sgt. Maj. Lawrence Boivin deployed five times, but his final act was to shove his wife to safety at the expense of his own life.
"I saw the train hit the float," witness Jaime Garza said, pausing with emotion. "I made a quick U-turn to get back up there. The first person who was there was Lawrence. I had to help him out. And he gave me his last breath right there."
Army Sgt. Joshua Michael also died saving his wife.
On Friday morning, Midland Mayor Wes Perry was among the mourners at a vigil honoring the train collision victims, according to the Midland Reporter-Telegram.
"In my opinion this is maybe the most tragic event Midland has ever experienced, and the only thing I know to do is to gather together, be together like family as we are family, and to support each other, support those that lost their lives and support their families," Perry said. "We will be stronger on the other side once we get through this."
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