MIDLAND, Texas – A parade float filled with wounded veterans that collided with a freight train had crossed onto the railroad tracks after warning signals were going off, investigators said Saturday.

Four veterans of Iraq and Afghanistan were killed and 16 more people were injured when the train crashed into the flatbed truck in West Texas.

It was the second of two floats carrying veterans in Thursday's parade in Midland. The first was exiting the tracks when the warning bells and signals were activated, 20 seconds before the accident, according to the National Transportation Safety Board. The second float didn't enter the tracks until several seconds after the warning system went off, the NTSB said. By that time, the guardrail was lowering.