D.C. police dredged the Anacostia River under the John Philip Sousa Bridge yesterday in search of a baby and a woman who reportedly threw the infant off the bridge, climbed over the railing and dove into the river.

A 32-year-old motorist, whom police refused to identify, told officials that he picked up a woman hitchhiking with a baby about 1:45 p.m. at a McDonald's restaurant at Barney Circle and Pennsylvania Avenue SE near the bridge, a police spokesman said. The unidentified man told police that he stopped for the woman and child because he was concerned that the infant was not properly dressed for the chilly weather.

The motorist told police that as he drove southeast on Pennsylvania Avenue over the Sousa bridge, the woman became agitated and asked him to let her out of the car.

When he refused to stop the car, police said, the woman struck him with her hand, causing him to slow to nearly a stop. Police would not say why the woman had become agitated or why the man had refused to stop the car.

After the car slowed on the bridge, the woman jumped from the car, went over to the guardrail and tossed the infant into the river then jumped in, the motorist told police officers.

No bodies were recovered during yesterday's search. Police said they have no reason to doubt the motorist's account, but questions were raised about some of his actions and the lack of other witnesses.

"We believe his statement to be correct," said Capt. William White III, a police spokesman. "We have been unable to locate anyone else who observed the incident.

"But we have no reason to doubt his statement based on our investigation that involved us conferring with a relative and employer."

The motorist is employed as a security guard for International Security Corp. and works at a senior citizens home at 3333 Wisconsin Ave. NW, sources said.

A police source said, however, there were troubling questions about the motorist's account, and the man was taken into police headquarters for questioning.

Police said the motorist apparently drove about eight blocks away to his home on S Street SE before calling police to report the incident, instead of calling from a pay phone or business closer to the Sousa bridge.

Although the traffic was heavy on the Sousa bridge, police were unable to find any other witnesses to the incident. Three workers on the western bank of the bridge about 75 yards away did not see the incident, police say.

"We have to treat this is as a serious incident," said the police source. "But there are some questions."

A cook at the McDonald's restaurant said that he saw a woman and a child about 1:30 p.m., and the woman asked for food but had no money. The cook said he could not remember any other details about the pair.

Three police harbor boats and one fire department boat dragged the Anacostia River for about three hours yesterday in the rain. A strong current made the search difficult, police said.

The river is 4 to 6 feet deep at the point the woman allegedly jumped because the tide was going out. That point is about 50 yards from the eastern shore. The Sousa bridge is about 40 feet above the river, police said.

White said the motorist described the woman as about 40 years old, 5 feet 9 inches tall, 140 to 160 pounds and wearing blue jeans, a dark-colored top and tennis shoes.