Four people are dead after a medical helicopter crashed in a national forest in Texas, AP reported. Federal aviation spokesman Roland Herwig says the crash happened early Sunday. The helicopter was on its way from a hospital in Huntsville to one in Houston. Herwig says authorities do not know what caused the crash. Rescue teams had trouble finding the crash site in the Sam Houston National Forest. The helicopter belonged to Louisiana-based PHI Inc. Herwig says federal safety investigators are headed to crash site about 70 miles (113 kilometers) north of Houston.