Wearing black tape across his gold D.C. police badge, D.C. Deputy Chief Isaac Fulwood sat at his desk yesterday afternoon and struggled with his emotions as he mourned the line-of-duty death Tuesday of one of his officers.

"You ask yourself why," Fulwood said in a low, shaking voice. "A good, young officer, married with two very young children dies at the prime of his life . . . "

With tears streaming, Fulwood excused himself during an interview to splash water on his face, then returned. "All over a stolen auto," he said.

Fulwood commands the 6th District, where Officer Donald G. Luning, 31, had worked for 10 years. Fulwood and his men were wrestling with grief yesterday after the slaying of Luning, who was shot once in the chest with his own gun Tuesday afternoon during a struggle with the driver of a stolen car.

Police said that Luning was with his partner about 4:20 p.m. Tuesday when he spotted the stolen car in the 3700 block of Jay Street NE. The driver parked the car and fled into an apartment building in the 3700 block of Hayes Street NE.

According to an affidavit presented by the U.S. attorney's office in D.C. Superior Court yesterday, Luning cornered the suspect in the apartment building and was about to handcuff him when the man, described as weighing 220 pounds and standing 6 feet, 4 inches, resisted and "gained the upper hand."

During the struggle, the affidavit said, Luning's gun fell to the floor and the suspect got on top of the officer, who was lying on his back, and held him down. The suspect then picked up the gun, stood over the officer and shot him.

Police have arrested Mark Anthony Watson, 19, who told the court that he lived on Edgewood Street NE. Police had given out a different, and incorrect, address for the suspect Tuesday.

Watson was arraigned yesterday on a charge of first-degree murder and ordered held without bond. At the same time, prosecutors dropped auto theft charges against a woman riding with Watson after determining she was unaware that the car had been stolen.

Prosecutors said that Watson is a high school dropout who has an extensive juvenile record, is wanted in Fairfax County on a burglary charge, and has a firearms charge pending against him in the District of Columbia.

Meanwhile yesterday, Heroes Inc., an organization that provides financial support for the relatives of Washington area policemen and firemen killed in the line of duty, gave $2,500 to Luning's widow. The Fraternal Order of Police added $1,000. She will also receive $50,000 from the federal government and widow pension benefits from the city.

In the 6th District yesterday, officers put black tape across the badges on their police cars, as well as their uniforms. Said master patrolman Frederick D. Johnson, "It always seems like something like this always happens to the best."