The Orange County Grand Jury will begin hearing testimony today on the shooting death of a 20-year-old man last month by an off-duty federal officer during a confrontation at a noisy party, Dist. Atty. Tony Rackauckas said. Sheriff's officials said U.S. Customs and Border Protection Officer Douglas Bates killed Bassim Chmait with a single shot to the head during an altercation Feb. 5 at a Mission Viejo apartment complex. Authorities said Bates, who lives in the complex, got into a fight with Chmait, of Laguna Hills, and four to six of Chmait's companions as they arrived at the party about 1:30 a.m. The initial report of the incident by the Sheriff's Department said Bates came out of his apartment because he heard a disturbance and that his duty weapon discharged, perhaps accidentally. The victim's brother, Omar Chmait, challenged the official version. Omar Chmait was not present, but he has said that witnesses, who were friends of the victim, contended that Bates had provoked the fight. Law enforcement officials familiar with the investigation said several witnesses gave contradictory accounts of the shooting. On Thursday, Rackauckas said the grand jury would sort through the testimony and decide whether to indict Bates.