Four people suffered minor injuries Thursday morning when a semi-truck collided with an ambulance and then struck two other vehicles.

The American Medical Response ambulance, which was not carrying patients, received severe damage, including a broken windshield.

The accident occurred at 8:31 a.m. at Francis Street and Mountain Avenue, which police blocked off for about two hours as the damaged vehicles remained scattered nearby.

Truck driver Rafael Vazquez, 40, of Ontario, said he did not hear the ambulance and thought it would stop before the collision occurred.

"I didn't hear the noise from the ambulance," Vazquez said. "The ambulance had slowed on the street. I put the brakes all the way down."

The ambulance was on its way to help the Ontario Fire Department during an emergency call for service in the 1300 block of West E Street, police said.

Ontario police Officer Dereck Anthony said traffic on Mountain was stopped for the eastbound ambulance, which had its lights flashing and sirens on, as it approached Francis.

After striking the ambulance, the truck hit a stopped Jeep Grand Cherokee as well as a Toyota Camry.

Police said the ambulance's driver and passenger were transported to a local hospital, as were the Grand Cherokee's driver, who is pregnant, and the Camry's driver, who was described as an elderly female.

Their names were not released.

Anthony said no one had been cited in the accident and it is under investigation.

Alcohol or drugs do not appear to be involved, he said.