Authorities are investigating the cause of a head-on collision that killed five people on the Pacific Coast Highway in Ventura County early Sunday morning.

According to the California Highway Patrol, a blue Subaru WRX driven by a man in his 30s and an SUV with four occupants collided around 4:30 a.m. near Mugu Rock.

Both vehicles caught fire after the crash, and all five perished inside their vehicles, Ventura County Fire Department Capt. Brian McGrath confirmed to the Ventura County Star.

“They put out the vehicle fires and following that, we located five fatal victims in the two vehicles, four in a small SUV, which is the main car that caught on fire, and another solo fatality in a blue Subaru,” said Sgt. John Larson of CHP Ventura.

CHP says one of the vehicles apparently veered into oncoming traffic, although it was not clear which one.

“It’s highly unusual to have this many fatal victims in a vehicle crash like this,” Larson added. “A lot of these crashes, there’s multiple causes, but oftentimes, it comes from somebody speeding or drunk driving, one of the two.”

Both directions of the PCH were closed as of 8 a.m. Sunday morning and investigators were remained on the scene for several more hours.

“Really bad accidents happen on the PCH all the time, head-on collisions, people going too quick around the corners,” West Hollywood resident Natasha Florez , who had been forced to find a different route to her destination, told KTLA.

Naval police were also on scene, and though it is unclear how they are connected to the crash, Naval Base Ventura County is nearby.

As of Sunday night, investigators were still working to notify family members of those who were killed and were not releasing the victims’ identities.

Officials with the CHP were also urging anyone who may have witnessed the accident or have information about it to contact investigators in Ventura County.