A Columbia high school senior, charged in what a prosecutor called the "completely ghoulish" stabbing death of a convenience store clerk, was ordered held without bail yesterday at Howard County jail.

In other developments in the weekend slaying of Angelica Velazco, Howard County police are trying to determine if the arrest of David A. Hartless, 17, might yield any clues in the unsolved 1985 slaying of his grandmother.

Hartless' grandmother, 75-year-old Zulu Brown, was found strangled in the back of her car at a Columbia shopping center on March 28, 1985, police said. Her body was discovered two days after she was reported missing by Hartless' mother, and her unlocked home showed no signs of forced entry.

Sgt. Angus Park said that although Hartless was not considered a suspect in the death of his grandmother, detectives had decided to reopen the case based on the charges against him.

The homicide was the third in the city of 68,000 since 1984.

Hartless has been charged as an adult with first-degree murder and robbery in the Saturday afternoon slaying of Velazco, 20, a cashier at a High's store in a busy shopping center just blocks from Wilde Lake High School, where Hartless is a senior.

The youth was arrested at a friend's home Sunday, a little more than 24 hours after a customer discovered Velazco's body. She had been stabbed repeatedly in the abdomen and arms and almost decapitated in a back room at the High's store, Park said. After news of the slaying spread, several witnesses reported having been waited on by a young man, with bloodied hands, matching the suspect's description.

Park said the viciousness of the slaying, its location in an area not known for much crime, the apparent lack of a struggle and signs that the suspect apparently knew how to operate the cash register led police immediately to speculate that "this was more than a simple random robbery . . . . "

Hartless was once a clerk at a High's in Clarksville and he and Velazco were acquaintances, Park said.

In arguing that Hartless should be released on his own recognizance to his mother's custody, his attorney, James B. Kraft, said there was no reason to suspect that the youth posed a threat to the community.

Assistant State's Attorney Gerard Hogan said that among the police evidence was a dollar bill bearing a bloody fingerprint that apparently matches Hartless'.

After denying bail to Hartless, District Court Judge R. Russell Sadler recommended that he undergo psychiatric testing at the Clifton T. Perkins Hospital.