DIMMITT, Texas (AP) — A fire and explosion at a dairy farm in the Texas Panhandle that injured one person and killed an estimated 18,000 head of cattle was an accident that started with an engine fire in a manure vacuum truck cleaning part of the massive barn, according to state investigators.

A State Fire Marshal report of the April 10 fire at the Southfork Dairy Farm about 50 miles (80 kilometers) southwest of Amarillo did not determine what caused the engine fire, but found there was “no intentional act to cause a failure." The April 24 report said the investigation was closed.

A worker driving the truck through a section of the 2 million-square-foot barn told investigators he noticed what he first thought was steam coming from the engine compartment. He tried to drive out of the barn when he realized it was a fire, but couldn't make it.

The driver said he tried to put out the fire with two extinguishers but failed. Several other employees told investigators they rushed to help, but the fire quickly spread.

The report noted the dairy farm had a second manure hauling truck on the property outside of the barn. A dairy manager told investigators it too had previously burned. The report noted burn marks near the engine compartment consistent with the truck fire inside the barn.

According to the Animal Welfare Institute, the number of cattle killed made the incident one of deadliest farm blazes recorded since it began tracking fires in 2013.