Michael J. Gardner, a former Pentagon guard, was convicted of first-degree murder yesterday in the May 10 shooting of an Arlington man -- a crime that county prosecutors said was motivated by jealousy over a woman.

The Arlington County Circuit Court jury recommended a sentence of 25 years for Gardner, 31. Judge Thomas R. Monroe, who under Virginia law can lower the sentence but not raise it, set sentencing for Dec. 7.

If Gardner receives the full sentence, he would be eligible for parole in five years, according to assistant prosecutor Ruth Jamison.

Gardner was found guilty of shooting Orlando C. Dominguez, 22, in the corridor outside an apartment where Dominguez was living with Gardner's former girlfriend. Defense attorney Irving Starr claimed that Gardner fired his gun in self-defense after Dominguez ran toward him in the corridor holding a knife.

The verdict from the jury of seven men and five women came after nine hours of deliberation and an earlier announcement of a verdict that turned out not to be unanimous.

Shortly before noon, after deliberating for four hours Thursday and two-and-a-half hours yesterday morning, the jurors submitted a written question to Monroe asking whether they were required to reach a unanimous verdict. Monroe replied that they were.

Less than 30 minutes later the jury returned with a verdict, finding Gardner guilty of first-degree murder and recommending a 25-year sentence. But when individual jury members were polled aloud, a routine procedure, one juror said he did not agree with the verdict.

Monroe first declared a mistrial, then, responding to prosecutors' arguments that the jury needed more time to reach an agreement, reversed that decision. Speaking solemnly to the jurors, he urged them to continue deliberating.

"Somebody has to decide this case," he said, adding that a retrial would be less fair because witnesses' recall of the incident would fade.

The jury returned about two hours later with the same verdict as before. When polled aloud, all 12 said that they agreed.

Gardner, dressed casually in khakis and a tan velour shirt, remained expressionless both times the verdict was read. Carla Keastead, Gardner's former girlfriend, sat alone in the rear of the courtroom, her face puffy with tears.

Defense attorney Starr said that he planned to file motions to set aside the verdict, citing the initial reading of a decision that was not unanimous.