The stepfather of a Columbia teen-ager charged with slaying and robbing a convenience store clerk last month also was charged with murder in the case yesterday as well as with conspiring from his prison cell with his stepson to hold up the store, Howard County police said.

Investigators said Leo John Rites, 40, who was in the Maryland Penitentiary in Baltimore at the time the High's Dairy Store in Columbia was held up Oct. 17, is accused of plotting the robbery with his stepson, David Hartless, 17. Rites was charged with first-degree murder because a cashier at the store, Angelica Velazco, 20, was slain during the robbery. A source said that the holdup was conceived as a way to pay off drug debts accumulated in prison.

Velazco was stabbed repeatedly in the abdomen and arms and almost decapitated in what a prosecutor called a "completely ghoulish" slaying.

Hartless already had been charged as an adult with murder and robbery. He is being held without bond at the Howard County jail.

Since May 1986, Rites has been serving a 30-year armed robbery term at the penitentiary in Baltimore, where he was arrested yesterday. In addition to murder, he was charged with conspiracy to commit robbery with a deadly weapon.

Friends and neighbors had painted Hartless as a friendly and somewhat outgoing Wilde Lake High School senior with a history of family problems. They had emphasized that the youth was very close to his stepfather and was upset when his parents were divorced last month.

Linda Rites, Hartless' mother, said in a telephone interview yesterday that she was shocked by the arrest of her former husband. "I'm stunned," she repeated several times. "I have no knowledge of this. I have to sit down on this one. You have taken me on a loop."

Hartless' attorney, James Bernard Kraft, declined to comment yesterday. Prosecutors are scheduled to present the Hartless case to a Howard County grand jury today.

Leo Rites said during a brief hearing last night that he will represent himself in the case. Rites is scheduled to have a bond hearing today. Hartless and Rites each face the death penalty on the murder charge and 20 years in prison if convicted of the robbery charge.

Police said during the hearing last night that Rites has lived in Columbia since 1965 and his last job was at a bagel shop in the Wilde Lake Village Center.

Sgt. Angus Park, a police spokesman, said Rites was arrested at the penitentiary at about 2 p.m. yesterday and was taken to Ellicott City. He was scheduled to be taken before a District Court commissioner late last night for arraignment on the murder charge.

Park said Rites' arrest was based on evidence police seized in search warrants and interviews with people who said they knew of the "close relationship" between Rites and Hartless. Linda Rites, in an interview last month with The Washington Post, said her son was particularly fond of her fourth husband, almost to the point of "worshiping him." Rites said the youth worried about what would happen to his stepfather in prison.

According to Park, police confiscated "documents" allegedly linking Rites to the robbery and murder from Hartless' high school locker, a friend's home and Rites' prison cell. Park refused to say what documents were seized. Authorities said the robbery was a "joint plan" by Rites and Hartless. Park said he did not know when the alleged plot was concocted, but "there was plenty of opportunity for contact."

Park said he does not expect additional arrests in the case.

During a hearing at which Hartless was denied bail, a prosecutor said that police had found a dollar bill with a bloody fingerprint that has been linked to Hartless. Last month, police found a bloodstained denim jacket in a wooded area near the suspect's home. Authorities are awaiting lab results on the jacket. The murder weapon has not been found yet, a source said.

Hartless, who once was a clerk at a High's store in Clarksville, apparently knew the victim, police said. During recent months, Hartless frequented the High's store in Columbia, where Velazco worked part-time while training to be a cosmetologist.

Staff writer Amy Worden contributed to this report.