WASHINGTON (AP) _ A helicopter being used on a photography assignment crashed into the Potomac River on Friday, fatally injuring three passengers. The pilot escaped by apparently leaping from the plunging craft.

Dozens of people at a nearby marina saw the Bell Jet Ranger hover about 15 feet above the water and then suddenly plunge into the Washington Channel section of the river around 7:30 a.m.

Assistant Police Chief Isaac Fulwood Jr. said unspecified ''mechnical difficulties'' apparently caused the helicopter to crash.

''It started to go in nose first, then the pilot was able to level it slightly - and then it splattered,'' said Marilyn Sharkey, 63, who lives on a boat in the marina. ''It went straight down on its belly ... It broke up into nothing.

''The pontoons came up, and about three or four seconds later, an arm,'' she said. ''Quite a few of the boats went out to see if they could help.''

One of the men killed, William Weems, 44, a freelance photographer, had leased the helicopter from Stewart Aircraft Corp. to take pictures for a brochure for the Arlington, Va.-based Sumner Realty Corp., said Turner.

The other two victims were identified by hospital officials as Robert Arthur Joy, 45, of Washington, and Victoria Hinckley, 24, of Rockville, Md.

Weems died of cardiac arrest due to drowning, Hinckley succumbed to chest injuries, and Joy died of multiple injuries and cardio-pulminary arrest, hospital officials said.

The lone survivor, pilot Jack Turley, 37, of Baltimore, ''apparently was able to jump from a helicopter as it hit the water,'' Fulwood said. Turley was listed in serious but stable condition at George Washington University Medical Center with head and spinal injuries, said spokeswoman Katharine Baker.

Civilians already were bringing Turley to shore in their boat when rescue workers arrived on the scene, said Police Chief Maurice Turner.

Police harbor boats arrived within three or four minutes, but because they aren't equipped with diving gear, ''they had to go back down to the harbor to pick up equipment and personnel,'' delaying the rescue at least 15 minutes, Turner said.

''Certainly, in light of what transpired, we will be making a decision about putting scuba gear on the harbor boats,'' he said later.

An Army Corps of Engineers crew lifted the submerged helicopter from the water using a crane mounted on a barge. The craft was released to the National Transportation Safety Board for examination.

Twenty-three of the 118 helicopter accidents in the United States this year have involved the Jet Ranger, NTSB officials said. Seven of those crashes were fatal, resulting in 16 deaths.

The Jet Ranger, which is considered ''a very reliable aircraft,'' is used for a broad range of functions, from transporting corporate executives to crop dusting, said Ken Woolnough, safety director of the Helicopter Association International.