The discovery of a .22-caliber rifle and a makeshift shooting range in the basement of the McLean home where a World Bank official and his wife were shot to death Monday night led to the arrest Thursday of their son, Alain Paul de Cock, in the slaying, according to Fairfax County Circuit Court records.

After telling police repeatedly that he didn't own a gun, de Cock, 21, told them Wednesday that he "had lied," according to an affidavit filed with the court. De Cock said he had illegally brought a .22-caliber rifle into the United States when he moved here with his family from Rome, the affidavit said.

A report by James C. Byer, Virginia deputy chief medical examiner, indicated that Belgian nationals Romain Paul de Cock, 52, and Simone Irene de Cock, 50, died of gunshot wounds in the head and that the wounds were believed to have been caused by .22-caliber bullets.

De Cock told police that the rifle was in the bedroom closet of his parents' home, where he also lived. After several searches, police found the weapon Wednesday tucked into a crawl space in the basement of the $200,000 home in the exclusive Langley Oaks subdivision.

Investigators also found a bullet-riddled Maryland telephone directory, several small targets and bullet holes in the walls, according to court records.

De Cock was arraigned yesterday in the Fairfax Juvenile and Domestic Relations Court on two charges of murder and two charges of using a firearm in the commission of a felony. De Cock entered no plea to the charges.

Judge Thomas A. Fortkort ordered de Cock held without bail in the county adult detention center. A thin young man with tousled brown hair, Alain Paul de Cock was little known in the comfortable neighborhood of international residents where he had lived with his parents since the spring of 1981.

Although neighbors said young de Cock spoke several languages and had lived in Rome and Zaire, he seemed a loner they knew mostly for playing with the family dogs, riding his motorcycle or chain smoking cigarettes on the house deck which he had helped his father build last summer.

He had also helped his father build a trading-post fence around the property and worked with him in the garden, neighbors said. But, said one neighbor, the father and son argued frequently. "Mrs. de Cock was worried that her son and her husband fought, but she didn't know what to do," the neighbor said.

A neighbor who car-pooled to work with Romain de Cock said he rarely talked about his son. "It seemed strange for a father not to discuss his son," the neighbor said, "but I figured he felt it wasn't appropriate among the people with whom he worked."

The neighbor said Simone de Cock became increasingly protective of her son after her teen-aged daughter died of cancer last year and wanted him to stay at home and attend a communnity college rather than go away to school. While neighbors said they believed Alain de Cock to be enrolled in a community college, schools in the area had no records of his attendance.

"Mrs. de Cock would frequently talk about how in Rome, her daughter, even though she was sick, would have friends over for parties," a neighbor recalled. "She was concerned that Alain rarely brought friends to the house."

Young de Cock was arrested on Nov. 13 for driving while intoxicated, according to records in Arlington General District Court. The case has been scheduled for trial Jan. 26.

It was Alain de Cock who first reported the deaths of his parents to officials. His mother was found shot to death upstairs in her bedroom and his father downstairs in the foyer. Police said the murders occurred between 6 p.m. and 11 p.m., shortly before Alain de Cock called rescuers to the scene.

De Cock appeared alone in court yesterday, wearing blue jeans, a wrinkled blue shirt and a suede jacket. When the judge asked if he had retained a lawyer, de Cock replied, "Last night they told me someone would be here for me."

Minutes later, D.C. lawyer Susan Sinclair, accompanied by two men who apparently spoke little English, appeared in the courtroom. Identified as de Cock relatives, the men, speaking through Sinclair, said they had not determined whether they would retain counsel for de Cock.

The judge ordered de Cock to return to court at 9 a.m. Monday to report on the status of his legal representation. If necessary, the court will appoint an attorney for him. The arraignment was scheduled in Juvenile and Domestic Relations Court because of a Virginia law that requires all homicides involving family members to be held in that court. De Cock will be tried in the county Circuit Court.Washington Post staff writers Nancy Scannell