A Fairfax Circuit Court judge refused yesterday to block prosecutors from using Alain Paul de Cock's statement to police that he killed his parents in the family's McLean home, rejecting defense motions that police illegally coerced the statement.

De Cock's court-appointed attorney argued that police investigators prodded the 21-year-old suspect to implicate himself in the December slayings by implying that he could get a more lenient sentence if he cooperated with police. Such an action would be illegal under Virginia law.

After the three-hour hearing in which a tearful de Cock took the stand in his own behalf, Judge Barnard F. Jennings said he found no violations of law and "no evidence of any threat or duress" used to obtain the statement.

Defense attorney Robert C. Whitestone said the judge's ruling has severely crippled de Cock's case: "With the confession allowed, it is only remotely likely that we could be successful in defending the case."

De Cock has been charged with two counts of murder in the Dec. 13 shooting of his father, Romain Paul de Cock, 52, a World Bank loan officer, and his mother, Simone Irene de Cock, 50, at their house in the affluent Langley Oaks subdivision of McLean.

Whitestone said de Cock will plead not guilty to the charges at his trial Feb. 28.

A distraught de Cock testified yesterday that he would not have implicated himself in the killings if he hadn't felt police investigators were insinuating that the statement would result in a lighter sentence.

The thin, pallid youth said that when he returned to the police station for his second polygraph test three days after the shootings, a police investigator told him " 'I know you did it. I just want to know why.' . . . He said I'd feel much better if I got everything off my mind."

Choking back tears, de Cock said the investigator "said he thought I was a nice guy, that I had a brilliant future ahead of me . . . Then he told me about the case of a young man in a similar situation who was set free about one and a half years later."

De Cock added, "I took it to mean I could be out in about two and a half years."

Commonwealth's Attorney Robert F. Horan Jr. argued that the investigators, responding to de Cock's questions, simply described to him the full range of possible penalties, from the death sentence to a minimal jail term, and made no promises.

He said de Cock's only request during the questioning "was to ask, 'Can I see my dog before I go to jail?' "

Citing testimony from police investigators, Horan said de Cock gave the statement freely after police discovered his rifle, believed to be the murder weapon, hidden in a crawl space in the basement of the family's Ridge Drive house.

In earlier court hearings, police said de Cock told them he shot his Belgian parents because he felt unloved and rejected.

He said friction had been increasing between him and his parents since his younger sister's long illness and death from cancer a year ago, police reported.

De Cock shot his mother in the head after a daylong argument, according to the statement he gave police. A short time later he shot his father in the head as the elder de Cock stepped into the foyer upon his return from work, according to the statement. It is that statement that Judge Jennings refused to throw out yesterday.

Neighbors and acquaintances have described the younger de Cock as a loner who had made few friends in the United States since moving here with his parents from Rome three years ago. They said he spent much of his time at home playing with his dog.