In a hushed Montgomery courtroom yesterday, lawyers and observers listened attentively to a tape of an emotional call to the county's 911 emergency line from a young man describing the shooting of his father.

At 6:58 p.m. on Jan. 22, Patrick Joseph O'Sullivan Jr. dialed the 911 emergency operator to report that his family's Rockville dry cleaning business had been robbed and that his father was pursuing the thief down Muncaster Mill Road.

O'Sullivan, 24, said that he was keeping in touch with his father on the two-way radio installed in his father's white pickup truck. He said his father had tracked the robber to Cactus Court in Gaithersburg and the gunman had approached his father as he was sitting unarmed in the truck.

"He's trying to shoot my father," O'Sullivan told the 911 operator. And then there were sounds of gunfire on the tape.

"He just shot him. Get an ambulance up there," O'Sullivan said.

Among those listening to the tape recording was Gregory William Wamsley, 26, of Derwood, who yesterday pleaded guilty to charges of robbery and murder in the death of Patrick Joseph O'Sullivan Sr., 50. Wamsley is scheduled to be sentenced Jan. 10 and faces a maximum term of a life sentence plus 60 years, said Circuit Court Judge Calvin R. Sanders.

Members of Wamsley's family, including his mother, left the hearing with tears in their eyes.

Wamsley sat yesterday in the courtroom with eyes downcast and shoulders slumped as he listened to the tape recording and the other evidence that had been gathered by investigators.

Wamsley, a former prison guard, was arrested by Montgomery County police within 15 minutes of the shooting. At the time, he was free on bond while waiting to begin a sentence in federal prison for smuggling marijuana into the Montgomery County detention facility when he was working there as a guard.

The Montgomery County state's attorney's office filed a petition in July declaring its intention to seek the death penalty in the murder case against Wamsley. But Assistant State's Attorney John McCarthy withdrew that petition yesterday after Wamsley agreed to plead guilty to the murder and robbery charges against him.

Patrick O'Sullivan took the witness stand to help narrate the tape recording of his father's death. He recounted how he was sitting at a desk in the Nu-Look Cleaners, 7213 Muncaster Mill Rd., Rockville, that evening when he realized that a gunman had entered the store.

The man, wearing a ski mask and carrying a revolver, ordered O'Sullivan's father, who was standing at the cash register, to empty the contents into a Giant shopping bag. The robber then told the father and the son to empty their pockets of cash. Altogether, he took $228.10, according to the indictment.

When the robber had left, Patrick O'Sullivan Sr., a former District of Columbia policeman, snatched up the keys to his truck to pursue the thief and told his son to call 911, the son told the court yesterday.

McCarthy also introduced into evidence a stack of bills investigators took from the glove compartment of Wamsley's car. They indicated Wamsley was under financial pressure, McCarthy said.