An 18-month old baby was killed and three adults were injured early yesterday afternoon when a houseboat headed up the Potomac River exploded and burned 500 feet north of the Woodrow Wilson Bridge.

A series of three explosions quickly tore the 30-foot fiberglass boat apart at about 1:45 p.m., engulfing the boat in flames and dense black smoke. The three adults climbed in the 40-degree river and escaped.

Investigators could give no explanation for the explosions, which were visible from both sides of the river.

"There was a sound like thunder, and then a big ball of smoke," said James Anderson, an Alexandria Red Cross employe who was at Jones Point under the bridge when the accident occurred. "I saw the explosions and then I saw three people, two men and one woman, climb down into the water."

According to witnesses, the adults, William Kline, 33, his wife Jeanette, 27, both of 313 E. Oak St. in Alexandria, and Steven Rhodes, 32, whose address is the Gangplank Marina in SW Washington, clung to ropes hanging off the boat.

Fairfax County fire officials, on a training exercise at Jones Point, repeatedly called through a megaphone to the three adults to get away from the boat, which was positioned in the middle of the river, after the first or second explosion. Witnesses said the boaters may not have heard and were still clinging to the ropes when a third explosion wracked the vessel.

Within minutes a private boat pulled Jeanette Kline from the river, and a D.C. Police boat picked up the two men, witnesses said. The three, who were returning from a cruise to Mount Vernon, were taken to the Alexandria Hospital, where they were listed in stable condition yesterday evening.

A hospital spokesman said Jeanette Kline suffered a fractured ankle and first, second and third degree burns. Her husband suffered unspecified burns and Rhodes was admitted for smoke inhalation.

D.C. harbor police and Coast Guard personnel continued to search the area for 18-month-old Arin Justin Kline, whose burned body was found in the boat's charred hull at approximately 2 p.m. He was pronounced dead at 4:10 p.m. by the D.C. deputy medical examiner.

The gutted and almost completely submerged houseboat, which belonged to Rhodes, was pulled to the Alexandria side of the river, where police moored it to a pier. It sank, leaving only its 12-foot wide bow above the water, which was covered with oil and filled with blackened bits of wood.

Harbor police said Rhodes had purchased the 11-year-old houseboat six months ago and it still carried two names -- "The Turtle" and "The Bounty."

Investigators pulled two fuel tanks intact from the flotsam. Both still contained some gasoline, police said.

Woodrow Wilson Bridge operator James Elrod said when he heard the first explosion he believed there had been an accident on the bridge. "I heard the explosion and thought it was on the bridge at first," he said yesterday. "I sat back down, and then I saw the black smoke. There was so much of it, it was hard to see anything.

"The boat was entirely engulfed in flames and burned down to the waterline. It was so quick as to be almost unbelievable that anything could burn that fast, because it was a rather large houseboat," Elrod said.

Although the portion of the river where the accident occurred is under the jurisdiction of D.C. Police, numerous police agencies responded to the explosion. Boats from nearby National Airport, the D.C. harbor patrol, and from Fairfax County went to the scene along with the Coast Guard cutter Capstan.