Even though Mark Francis Arban was diagnosed as a paranoid schizophrenic, he was able to legally purchase a military-style semiautomatic assault rifle with bayonet, under Virginia laws regulating gun sales, police said yesterday.

Arban used the high-powered rifle to kill Prince William County police Officer Philip Pennington on Thanksgiving after he and other police officers went to Arban's Dale City home with a search warrent. Pennington, 35, an 11-year member of the force, died after he was shot once in the head. A police sharpshooter then fatally shot Arban.

"He was a time bomb waiting to explode," Prince William County police spokesman Barry Barnard said about Arban. "Something needs to be done about people who have demonstrated violent behavior or who are mentally ill to the point that they have the capacity to be violent getting these firearms."

Arban had been prescribed an anti-depressant and a source said he was taking a second drug, the anti-psychotic Stelazine. Stelazine is commonly used to blunt delusions and hallucinations and to calm agitation. Medical experts say schizophrenics are no more prone to violence than other people.

Arban legally purchased an AKS-762 assault rifle and a .25-caliber Raven Arms semiautomatic pistol on Feb. 16, 1989, from Ted's Coins-Guns-Pawnbroker in Woodbridge, police said.

The store was closed temporarily last year and its owner, Donald P. Percival, and an employee, James S. Chapman, were jailed for selling guns illegally.

Arban, who has had counseling since his childhood, was under psychiatric care when he purchased the guns, according to police.

Statements Arban made during mental health counseling show he "felt extremely threatened by anyone in uniform," believed police were following him and felt the world was out to get him, Barnard said during a news conference yesterday.

Arban, 31, had lists of names of police officers and police car license plate numbers confiscated at his home by police in a search of his residence.

A search warrant shows that Arban, who had been unemployed for several years, also had $1,600 in cash at his home at the time of his death. Sources close to the case also said Arban had been arrested in the early 1980s in Prince William County for misdemeanor illegal possession of alcohol and on a weapons charge involving numchakus, two sticks attached by a chain or string used in martial arts fighting.According to Bureau of Alcohol, Tobacco and Firearms spokesman David Troy, anyone interested in purchasing a military-assault rifle such as the one used to kill Pennington, has only to fill out that agency's form 4473, a firearms transaction record.

The form asks if the potential purchaser is under indictment for a felony or has been convicted of a felony or is a fugitive. It also asks if the purchaser uses illegal drugs, but no mention is made of anti-psychotic drugs. The form does ask if the purchaser has been judged "mentally defective" or been committed to a mental institution.

Arban had no felony criminal record in Montgomery or Prince William counties, two places where he lived, and there are no records that he had been committed.

"If a person answers no to all those questions, they can buy whatever they want," said a Woodbridge gun store manager who asked that his name not be used. "And it's left up to them if they tell the truth. Nobody checks and nobody really knows."

Commonwealth's Attorney Paul B. Ebert said he is conducting a routine investigation into the shooting to determine if police followed departmental policy during the incident.

Barnard said the shootings are also under investigation by the Internal Affairs unit of the Prince William police, standard procedure after lethal force is used.