A car with four people inside plunged off a road into Los Angeles harbor, leaving a 13-year-old boy dead and an 8-year-old boy hospitalized in grave condition, authorities said.

A man and woman swam to the surface, but the two boys had to be rescued from the underwater car, and the older one was declared dead at a hospital a few hours later, authorities said.

"We don't know if the vehicle was driven into the water intentionally or accidentally," said Capt. Brian Whitten of the Los Angeles Police Department, which is taking over the investigation.

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

It traveled some distance before lurching into the harbor, where it landed upside down in about 30 feet of water, Fire Department spokesman Brian Humphrey said.

Divers found the 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 adults were described as being in fair condition but "clearly emotionally distraught," Humphrey said.

One witness said the man swam quickly to a ladder out of the water, and the woman was screaming as she came up.

"I thought she was drowning," witness Frank Palazzolo told the Daily Breeze newspaper. "She was going crazy. She was screaming, 'My kids, my kids, screaming real loud."

The relationships between the victims weren't immediately clear.

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

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