The National Transportation Safety Board (NTSB) is investigating last week's fatal accident in Midland, Texas, in which a Union Pacific freight train struck a flatbed semi-trailer being used as a parade float.

Four people were killed and at least 17 killed in the incident, which occurred when the flatbed truck participating in a “Hunt for Heroes” parade was struck by a Union Pacific Corp. train double-stacked with cargo containers. The train was traveling an estimated 55 miles per hour. Its two crew members were uninjured.

Texas-based NTSB investigator Robert Accetta is leading the team as the Investigator-in-Charge. NTSB Board Member Mark Rosekind is accompanying the team and will serve as the principal spokesman during the on-scene phase of the investigation.

In a briefing Friday, the NTSB said the flatbed which was stuck was following behind another one that also held veterans and their families. The first one made it safely across the tracks.

The parade was sponsored by Show of Support, Military Hunt Inc., a Midland-based group that holds an annual outdoors trip for wounded veterans, organizers said.

All the veterans involved had severe injuries that left them disabled.

Union Pacific spokesman Tom Lange said the company’s preliminary finding is that the crossing’s light and gates were functioning and that the train crew sounded the locomotive horn.