MEXICO CITY (AP) — Mexico's army said Friday the drowning death this week of four soldiers was caused by an accident involving confiscated weapons.

The patrol had picked up three seized assault rifles and were laying them in the bed of the patrol truck when one went off.

The round hit and killed the driver of the truck; the vehicle then went off the dirt road and plunged into an irrigation canal.

The three national guardsmen and a soldier were trapped under the vehicle and drowned near the Rio Grande around Reynosa, across the border from McAllen, Texas.

The army said an investigation into the incident was pending.