Fairfax County police yesterday charged 21-year-old Alain Paul de Cock with murdering his mother and father with a rifle Monday evening at their McLean home.

De Cock is charged with killing his father, Romain Paul de Cock, 52, a World Bank loan officer, and his mother, Simone Irene de Cock, 50. He also has been charged with two counts of use of a firearm in commission of a felony, police said.

Police said they arrested the son about 4 p.m. yesterday after finding what was apparently the murder weapon, a .22-caliber rifle, at the de Cock residence at 739 Ridge Dr. in McLean. He was taken into custody after questioning at the police Criminal Investigatons Bureau headquarters.

As recently as Wednesday police officials said they had not found the murder weapon and said the son appeared to have an alibi for his whereabouts at the time of the shootings. Police would say only that they arrested the son after finding the rifle at the family residence.

The shootings apparently resulted from a "domestic problem," police investigators said.

The suspect was held without bond in the county jail. He is scheduled to be arraigned in General District Court today.

The de Cocks were found slain in their house in the affluent Langley Oaks subdivision about 11:15 p.m.

The son reported finding his father's body in the foyer of the house and telephoned a county rescue squad.

Police officials later summoned to the scene discovered Simone de Cock's body in a second floor bedroom. The de Cocks had been shot in the upper chest, police said.

The de Cocks, natives of Belgium, moved into the neighborhood almost two years ago, neighbors said.

Romain de Cock had been employed by the World Bank since March 1981, according to a bank spokesman who said de Cock was an agricultural loan expert who traveled extensively in West Africa.

The son lived with his parents, neighbors said. A teen-aged daughter, two years younger than the son, died of cancer shortly after the family moved into the area, police said.

Neighbors said the death created a heavy strain on the family and that the mother "seemed sad all of the time."

The discovery of the rifle in the house led to the arrest in a case that had baffled police, who said they found no signs of burglary or forced entry. Neighbors said they heard no gunshots or strange noises at the residence on the usually peaceful road, though one neighbor reported hearing one of the family's dogs barking loudly about 9:30 Monday night.

The son had been staying with relatives who had arrived in Fairfax from Nice, France, on Tuesday, police said.

Authorities said they have questioned Alain de Cock several times since the slaying.