REVERE – A 20-year-old city man who authorities say fired the shot that killed Revere Police Officer Daniel Talbot the early morning of Sept. 29, was arrested Friday.Robert Iacoviello, Jr., 20, was arrested at 2 p.m. based on credible evidence developed in the course of an ongoing grand jury investigation, according to District Attorney Daniel F. Conley.Iacoviello is expected to be arraigned on that charge Tuesday in Chelsea District Court.The State Police Detective Unit sought a warrant for Iacoviello’s arrest. He has been held at the Nashua Street Jail since the morning of Oct. 1 on weapons charges and a probation violation unrelated to this investigation.Several police sources told The Daily Item one week after Talbot’s death that Iacoviello and Jimmy Heang, 17, were people of interest in the investigation.Iacoviello is the second person charged in the case. Derek Lodie, 17, of Revere, was initially charged with being an accessory before the fact. He pleaded not guilty during his arraignment.Police also arrested Heang’s brother, Dararin Heang, 19, when police raided their home at 167 Cooledge St. on Oct. 1. Both brothers face drug charges and possession of an illegal stun gun.Since the shooting, police have scoured the area and questioned scores of people.Talbot was shot once in the head as he socialized with other off-duty officers in a parking lot behind Revere High School in the early morning of Sept. 29. Conley said a confrontation took place where an unidentified shooter fired on the officers, who then fired back.An autopsy showed that the bullet that killed 30-year-old Talbot was not the same caliber as weapons the other officers carried.Conley said he promised that on the day Talbot died that his office would apply the law to hold the person responsible for his death accountable.”Today, 40 days later, we are delivering on that promise,” he said. “We are not done yet and much work remains to be done, but we are moving ever closer to finding justice for Daniel Talbot. The investigation is far from over.”Conley said he expects additional charges will be issued from the grand jury, which has already heard from 19 witnesses and will continue to hear testimony.On the night of Sept. 28 and continuing into Sept. 29, Talbot, his fiancée, Connie Bethell, 28, and three other members of the Revere police department left Margarita’s, a Revere bar, and proceeded to the bleachers behind Revere High School where, according to Conley, they continued to drink and socialize.Lodie was cutting through the field when he came upon the officers.The events that happened next have been the subject of intense scrutiny by homicide prosecutors, State Police homicide detectives and grand jurors.According to Conley, evidence suggests that Talbot and his party exchanged words during a confrontation with Lodie, who left the scene through a hole in a nearby fence.Lodie then allegedly called Iacoviello and orchestrated the altercation that would soon follow.A short time later, Lodie returned to the scene and heated words were exchanged. Evidence then suggests that Talbot and Officer William Soto began to approach Lodie, when at the same time Iacoviello and two young men arrived at the scene.Iacoviello, armed with a 9mm handgun, allegedly fired the weapon and mortally wounded Talbot with a shot to the head.Officer Soto then discharged his weapon in response as Talbot’s fiancée and Officer Stacey Bruzzese called for help.Iacoviello, Lodie and the two men then fled the scene.Addressing speculation on the case, Conley said most of it has been responsible and some has been wildly inaccurate.”First and foremost, we have developed no credible evidence that Officer Talbot or any other member of his group was engaged in any illicit activity at all behind the high school beyond the minor infraction of drinking in public,” he said. “Second, State Police took over the investigation because as District Attorney, I have designated them to investigate all homicides