SAN PEDRO, Calif. -- Two small planes collided over the ocean just outside Los Angeles Harbor and plunged into the water Friday, prompting a massive search by dozens of boats and divers. As darkness fell there was no sign of survivors.

An L.A. County official told CBS Los Angeles a fishing boat crew reported seeing a small aircraft plunging into the ocean at about 3:14 p.m.

LAX's radar shows the blue and yellow aircraft slamming into each other at 3,100 feet and eventually vanishing from the radar screen.

Investigators had no immediate word on what might have caused the accident or the number of people aboard the two planes.

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Fire and lifeguard boats and rescue divers swarmed the area about two miles outside the entrance to the harbor, where water depths were 80 feet to 90 feet.

Debris that included the full tail number for one plane and partial number of the other was found, Los Angeles County lifeguard Capt. Ken Haskett said.

A Coast Guard helicopter equipped with night vision and two vessels planned to search about 200 square miles of ocean for survivors through the night, with a more extensive search resuming at daybreak, Lt. Jonathan McCormick said.

The nearest harbor entrance was closed to traffic while the search continued.

The crash site was a quarter-mile south of the Angels Gate light, a lighthouse at the San Pedro Breakwater that is on the National Register of Historic Places. The area is popular for flight students, and there were many planes in the crystal-clear skies at the time of the accident.

Pilots communicate at two different radio frequencies -- one for above 2,000 feet and the other below, said Reed Novisoff, chief pilot at Pacific Air Flight School.

"People are very diligent about reporting their positions," Novisoff said. "It's very safe out there."

Nonetheless, Friday's midair collision was not the first.

In 2001, four people died when two Cessna airplanes carrying instructors and students collided 1,000 feet above the harbor.

In 1986, two small planes flown by students collided. But the aircraft managed to return to their airports and the four people on board escaped injury.

Richard Garnett, chief flight instructor with the Long Beach Flying Club, said the pilots practice in an area that is 10 to 20 square miles and at altitudes ranging from 1,000 to 4,000 feet. On a typical day, there will be three or four planes in the air at the same time.

"So with the amount of activity, actually, I think we've been fortunate," he said. "We are really diligent. I don't know why, what happened in this situation."