Tony A. Mackall, 23, should be executed if he is found guilty in the shooting death of a Woodbridge service station attendant, a Prince William County prosecutor said yesterday on the opening day of Mackall's trial.

"If you reach the conclusion that the defendant is guilty of capital murder, you will have to make a decision as to death or life in prison," Commonwealth's Attorney Paul B. Ebert told the 12-member jury. "I am going to ask you at the end to put him to death."

Mackall, who was an escapee from a D.C. Department of Corrections halfway house, is charged in the death of Mary E. Dahn, a 31-year-old mother of two who was shot once in the head last Dec. 9 during a holdup of a Rte. 123 Shell station where she worked. He also faces charges in the shooting of Washington Times sports editor Michael Keating, which occurred 30 minutes after Dahn was shot.

Mackall has pleaded not guilty to the charges of capital murder, felonious use of a firearm in the commission of a felony and robbery.

In his opening statement, Ebert characterized the case as a brutal double shooting: Dahn and her husband Stephen were both working at the gas station, he said. Stephen was hanging Christmas decorations when he heard a gunshot, ran toward the booth where his wife was working and saw Mackall, who fired a shot at him, according to Ebert.

After fleeing, Ebert said, Mackall encountered Michael Keating. He shot Keating in the head once, Ebert said, then told Keating to "get up so he could be shot again."

"Wondrously, he survived," Ebert said. Keating is expected to join 29 other witnesses for the prosecution in the three- to four-day trial.

According to state officials, Virginia has 32 convicts on death row.

The day began with Mackall's defense attorneys, Ronald Fahy and Frederick A. Edwards, unsuccessfully seeking a continuance just before Mackall told Circuit Judge Herman A. Whisenant Jr. that he had not had ample time to prepare for the case with his counsel. Whisenant denied Mackall's request, citing the fact that he had been incarcerated for almost a year and that the case had been continued once before.

Last November, Mackall was being held at a community corrections center at 1825 13th St. in Northwest Washington for burglary and unauthorized use of an automobile.

He failed to return to the halfway house on Nov. 26, said Anthony Patterson, a D.C. correctional officer. After his arrest in the Prince William case, county and city officials said he had been missing from the house 11 days before a warrant was issued for his arrest a day before the double shooting.