". . . I thought he was going to shoot me and I fired the gun. And I didn't know whether I killed him. I wasn't trying to kill him . . . . "

With that testimony, Harlow Brian Sails, 21, stunned the jury in his murder trial in Upper Marlboro yesterday as he admitted shooting an off-duty D.C. police officer who had intervened in the robbery of a jewelry store.

Sails' statement came in the fifth day of the trial in Prince George's County Circuit Court after he had demanded to testify and, according to his lawyers, "concocted a story" about attending a party in the District at the time Officer Raymond Hubbard was shot to death at Iverson Mall last Feb. 8.

Under cross-examination by state's attorney Arthur A. Marshall Jr., "the bottom started to fall out" of Sails' story, according to defense attorneys Michael Gallavan and Edward Varrone.

The prosecutor then called a recess, during which, Gallavan and Varrone told a reporter later, Sails told them, "I screwed up," and agreed to return to the stand and tell the truth.

"We did rob Kay Jewelers and I was inside when the shooting started . . . . " Sails began. "I seen a friend of mine on the ground, seen the officer shooting at him . . . . I told him to stop, throw the gun down . . . . When he turned to aim at me, that is when I fired at him," Sails told the jury as gasps of surprise rippled through the courtroom.

"I was surprised at the confession ," said Marshall, who was making a rare courtroom appearance as a prosecutor in the trial. "I've never seen it except on TV."

Sails' confession surprised even his family. "It shocked me," said his sister, Diana Ford.

Hubbard, 28, was shopping in the mall when he attempted to stop several men fleeing from Kay Jewelers following a holdup. He was fatally wounded in an ensuing gun battle.

"It was an assassination," Marshall told the jurors during his closing remarks, in which he urged them to return a guilty verdict that could bring a death penalty.

Marshall described Sails as a "calm, cold, calculating murderer . . . a professional, a criminal who deserves absolutely no consideration from you."

Marshall said Sails deliberated over killing Hubbard when he walked over to the disabled officer and fired a final shot into Hubbards's back--the shot that the prosecution alleges was the fifth and fatal bullet, piercing Hubbard's heart.

Defense attorney Varrone said outside the courtroom that his client had "confessed to everything except what he didn't do, that is, kill Hubbard premeditatedly."

In his closing argument, Gallavan said Sails did not intend to kill Hubbard, but fired his gun in panic.

Gallavan described the incident as "an armed robbery gone bad." The defendant was, according to the lawyer, "a panicked, a frantic young man."

In light of the confession, Gallavan told the jury, "It's clear there is no longer any issue about what Sails did ." The question remains, he said, whether Sails fired the gun intentionally.

Circuit Court Judge Howard S. Chasenow instructed the jurors they must decide whether Sails is guilty of first-degree (premeditated) murder, murder during commissiom of a felony or second degree murder and whether he committed five related robbery and gun offenses. Either of the first-degree murder convictions would make Sails subject to the death penalty since the victim was a police officer.

The question of whether he should die in Maryland's gas chamber would be decided by a judge or the same or a different jury.

Sails was the fourth person to be tried in the case. Three other men received life sentences last summer for their part in the shooting and a fifth suspect is still at large.

As the jury filed out to deliberate shortly before 6 p.m., Sails' sister said, "I think they're going to be lenient on him. They didn't have anything on him. He just got up there and cleared his conscience."

The jury was still deliberating early today.