4 Killed in Medical Helicopter Crash in California

— -- Four people have been killed in a medevac helicopter crash in Kern County, California.

Just before 7 p.m. local time Thursday, the Skylife H-4 helicopter left Porterville Airport with a critical patient who was being transferred to San Joaquin Community Hospital in Bakersfield, Calif.

The Fresno County EMS dispatch center immediately contacted both airport towers after a routine safety call to helicopter crew was not answered. Dispatch then contacted the local fire department and sheriff’s office.

There were four people on board the helicopter: a pilot, a flight nurse, a flight paramedic and the patient.

An Air Ambulance spokesperson told the media at a press conference that officials were in the process of contacting the families of the victims.

“We don’t know exactly what the cause of the crash was at this point," the Air Ambulance spokesperson said.

The National Transportation Safety Board (NTSB) confirmed to ABC News that a crash investigation has been launched.