Updated details from FontanaHeraldNews.com:

While returning to the fire station for a call, the 2007 Ford F-450 San Bernardino County Fire-Paramedic Squad  was heading south on Sierra Avenue when it was struck by an Oldsmobile Alero that was heading east on Randall Avenue at 2:30 a.m., according to the San Bernardino County Sheriff Coroner's Division.

Heavy rescue unit personnel worked to extricate the Alero out from underneath the squad to render medical aid to its occupants.  However, the two occupants of the Alero were found deceased

From ContraCostaTimes.com:

Emergency personnel from other stations responded to remove the two Alero occupants. Sateki Latu, 21 and Talai Livai, 21, both from Fontana, were declared dead at the scene. The Fontana Police Department is investigating the collision.

Alcohol was found inside the Alero, according to a press release from Fontana police. It is not know if either person killed had been drinking.

From AP:

Two people were killed when the car they were in slammed into ambulance at a San Bernardino County intersection, injuring two firefighters.

County Fire Department spokeswoman Tracey Martinez said Saturday the medic squad was returning to its station in Fontana when the high-speed crash occurred.

Heavy rescue equipment was used to extricate the car out from underneath the ambulance. The two occupants of the vehicle were found dead.

Martinez says the two firefighters in the ambulance are hospitalized with non-life threatening injuries.

The crash is being investigated by Fontana police.