SYDNEY, Australia — Two American tourists were killed on Wednesday when their helicopter crashed near a coral-viewing site on the Great Barrier Reef off northeastern Australia, the authorities said.

The helicopter, operated by Whitsunday Air Services, was heading to a popular sightseeing pontoon along the reef about 40 miles north of the Whitsunday Islands.

Spectators already on the pontoon watched as the helicopter plunged into the water and the pilot desperately tried to pull a passenger from the wreckage.

“This is a traumatic experience for anyone in the situation there,” Inspector Ian Haughton of the Queensland Police said Thursday at a news conference.