A car plunged off a road into Los Angeles harbor on Thursday, and two children pulled from the submerged vehicle were hospitalized in grave condition, authorities said.

The car left a parking lot at the Ports O' Call entertainment area of San Pedro shortly after 6 p.m. and ran off the road at Berth 73 of the Port of Los Angeles.

The car traveled some distance before lurching into the harbor, where it landed upside down in about 30 feet of water just off of Berth 73, Fire Department spokesman Brian Humphrey said.

Two adults were found swimming in the water when rescuers arrived, he said.

Divers found two children inside the car. They had no pulses and weren't breathing when divers brought them up, and resuscitation efforts began immediately, Humphrey said.

The children were taken to a hospital "in grave condition, fighting for their lives," he said. The adults were described as being in fair condition but "clearly emotionally distraught," Humphrey said.

It wasn't immediately clear whether the adults and the children were related, he said.

Divers working in murky water searched the car and even opened the trunk, but they found no one else in it, Humphrey said.

The cause of the accident was under investigation. "We don't know at this time whether it was an accident or an intentional event," he said.

Nearly 100 firefighters, divers and helicopter and boat crews went to the scene, Humphrey said.