The cause of a helicopter crash that killed three and left three others with serious injuries remained under investigation Sunday.

The helicopter crashed Saturday morning over the west end of Santa Catalina Island, which lies about 20 miles off the coast of Los Angeles County.

Two men and one woman were killed immediately, County Supervising Fire Dispatcher Melanie Flores said. Their identities were not immediately released.

Two women and one man were taken by helicopter to mainland hospitals, authorities said. Two of the injured were in critical condition with broken bones and other injuries.

It was not immediately known why the chopper went down and federal investigators were sent to the island to probe the cause of the crash.

The helicopter shot flames from its exhaust pipe while it was over water, county sheriff's spokesman Steve Whitmore said.

American Airlines flight attendant Debora Hansen, who was taking a walk by a yacht club, saw the helicopter go down and helped with the rescue of survivors.

She said it went down in a huge explosion and some of the passengers were thrown from the aircraft. Hansen said she and another man began pulling the survivors away from the flames. One woman was able to crawl to safety but a male survivor couldn't move.

"He said, 'I've got broken legs,' and I said, 'I'm going to pull you anyway or you're going to burn up in the flames,"' Hansen recalled. "The ambulances arrived within seconds it seemed."

Hansen said her training as a flight attendant helped her stay calm, but she was still shaken.

The Eurocopter AS 350 chopper was a private tour aircraft from Island Express, officials said.

Federal Aviation Administration records show the helicopter departed from Long Beach and was scheduled to return to that city. The crash occurred less than an hour after the helicopter picked up passengers on the island.

The crash occurred near the Banning House lodge, which overlooks a popular boating spot.

Catalina Island is a popular weekend destination for boating, snorkeling and camping.