(CNN) -- In an effort to understand why a truck serving as a parade float crossed in front of a fast-moving locomotive last week -- killing four war veterans on the float -- federal accident investigators on Tuesday will recreate the accident conditions.
The National Transportation Safety Board will stage a locomotive and a truck at the railroad crossing in Midland, Texas, to measure the sight distances between the two. The test will answer the question: "What could be seen, when, (and) where?" board member Mark Rosekind said.
The test comes as investigators this week examine a wealth of information about the crash, including several videos that captured the moments leading up to impact. They also are reviewing interviews with the train's engineer and conductor and are making plans to interview the truck driver.
The incident happened late Thursday afternoon during the "Hunt for Heroes" parade to honor members of the U.S. military. The last flatbed truck in the parade was crossing the tracks when an eastbound Union Pacific train slammed into it, Midland police said.
The safety board said the grade crossing warning system -- consisting of a bell, lights and a gate -- operated as designed, giving a 20-second warning of the train's arrival. But the truck entered the crossing, and the gate lowered, tipping over flag stands on the float, safety board investigators said.
Some people were able to jump off the truck in time, witnesses said.
But first responders found two dead at the scene, and two others were later pronounced dead at Midland Memorial Hospital. Fifteen others were injured, some seriously.
Those killed were identified as Marine Corps 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.
The safety board did not speculate or hint at the cause of the crash, but instead cited a number of facts that appeared not to be factors. The train was traveling 62 mph, the safety board said, under the 70 mph speed limit.
No anomalies have been found in the grade crossing warning system, Rosekind said. Nor have investigators found any mechanical problem with the train's air brakes, nor defects with the tracks, Rosekind said.
Rosekind said the south traffic light at the grade crossing turned green 21 seconds before the train's arrival at the crossing, as designed, to allow traffic time to clear the crossing. At 20 seconds before the collision, the bells and lights on the mast activated, in keeping with the minimum time under federal law.
At 13 seconds before the collision, the gate started coming down, and a second later, the front of the accident truck crossed the north rail of the tracks.
The engineer sounded the horn nine seconds before impact. And at seven seconds before impact, the lowering gate struck flagpoles on the float.
The train's emergency brakes were applied five seconds before impact, but it took the train 75 seconds to come to a complete stop, Rosekind said.
Rosekind said investigators would remain on the scene through much of this week and said a conclusion to their work is months away.
Investigators are reviewing video from a forward-facing camera on the locomotive, and video from a dashboard camera on a police cruiser that was behind the parade float, Rosekind said. They are also examining information on a data event recorder on the train, which recorded the train's speed, horn, etc.
The grade cross where the accident occurred had been designated a "quiet zone." Typically, trains blast their horns a quarter mile before the crossing. But the city of Midland applied to designate the crossing a quiet zone.
Rosekind said alternative safety measures are provided in quiet zones. "Our investigators will now be looking at exactly what those safety measures were and whether they were in place at the time of the accident," he said.
The safety board will also be looking at the parade permits to see what conditions were placed on the parade, and whether those conditions were met.