A mother, her two children and a friend's two children were abducted at knifepoint Thursday as they waited in a car outside a doctor's office in Capital Heights, Md., but escaped unharmed four hours later at a gas station in McLean, Va.

The abductor, a young man wearing casual clothes, said repeatedly during the four-hour ordeal: "Keep driving, I need to think." This led Prince George's County Police and an FBI agent from the agency's Alexandria field office to speculate yesterday that the man, still at large, might be one of the 30 inmates who escaped Wednesday night from the Maryland House of Correction in Jessup, Md.

"I would think so, personally," said county police spokeswoman Betty Havert. "The escapees would need transportation."

A police spokesman added, "Three of the escapees we have captured so far have been arrested wearing street clothes, not prison garb. That leaves the possibility that he might have been an escapee."

Consuela Scurry, 21, of Hillside, Md., told police the incident began about 3 p.m. Thursday as she was waiting for a friend outside a Capitol Heights doctor's office. With her in the car were her 3-month-old and year-old daughters and her friend's 2- and 4-year-old sons.

A man with a knife approached the driver's side of the car and ordered Mrs. Scurry to drive away. The 11-year-old son of Scurry's friend saw the man entering the car with the knife and told his mother, who then called county police.

Havert said the man ordered Scurry "to drive into the District, and they drove around downtown and the metro area for three hours."

Then the man told Scurry to drive into Fairfax County, where he had her stop the car on the Caiptal Beltway and took over the driving himself.

As the car neared the Langley Exxon station on Rte. 123 in McLean, Scurry convinced the man that she and the children needed to use the station's rest room.

The man bought a full tank of gasoline at the station, then drove away when Scurry and the children didn't return to the car.

David Masemer, the station's manager, said Scurry told her the man had taken all but $5 of her money and had told her he "would get her" if she told policy what happened.

Scurry's car, a 1973 Oldsmobile, was found abandoned at 3:50 a.m. yesterday in the 4400 block of Quarles Street NE., in the District.