An attorney for Thomas L. Grogg, 16, being tried as an adult on a first-degree murder charge, said yesterday that Grogg "emptied" a gun into the head of a Lorton construction worker, but only after the worker had made sexual advances to Grogg's 16-year-old aunt.

Grogg, who lives in Fredericksburg, Va., has pleaded not guilty to first-degree murder, robbery and a firearms charge in the death of Hallie Hollis, 28, who was shot in the head nine times Oct. 20. The statement of his attorney, William B. Reichhardt, came on the first day of his jury trial in Fairfax County Circuit Court.

The prosecutor, Fairfax Commonwealth's Attorney Robert F. Horan Jr., drew a different picture of the slaying, telling the jury that the shooting followed a declaration by Grogg's brother, Robert Grogg, 19, that he didn't like Hollis' attitude.

Hollis, who commuted to Lorton from his home in Williamston, N.C., had picked up the brothers, Robert Grogg's girlfriend and the brothers' aunt, who were hitchhiking on I-95 south of Fredericksburg.

Reichhardt said that after stopping to buy beer, Hollis stopped the car at a rock quarry near Lorton and the group was drinking and listening to the radio. All except Thomas Grogg, Hollis and Grogg's aunt, who was "passed out" in the front seat, had stepped out of the vehicle when Hollis made advances to the aunt and ordered Grogg out of the car, Reichhardt said.

"Tommy reached for the gun [a .22-caliber pistol strapped to his shoulder] . . . and emptied it," Reichhardt said. Horan said the four hitchhikers had no money and wanted to go to Florida, although their immediate destination was in Arlington. After being picked up and stopping to buy beer, Robert Grogg announced that he didn't like Hollis' attitude and argued with Thomas Grogg over who would kill Hollis, Horan said.

Thomas Grogg won the argument and shot Hollis, then pulled his body out of the car and asked his brother to run over it to make sure Hollis was dead, the prosecutor said.

The four hitchhickers were arrested on Oct. 26, in Sarasota, Fla., five days after Hollis' car was found in Daytona Beach, Fla.

Robert Grogg pleaded guilty last week to first-degree murder, robbery and using a firearm in commission of a felony.