PITTSBURGH — City police have hindered an independent review of the fatal shooting of a homeowner by officers responding to a home burglar alarm, the district attorney said Thursday at a press conference.

Officers shot 57-year-old Christopher Thompkins after someone fired in their direction as they arrived at his front door at about 4 a.m. on Jan. 22, according to police.

Thompkin’s ex-wife, Brenda Richmond Thompkins, who was staying with him at the time, said Thompkins grabbed her gun because an intruder had come in his home.

Allegheny County District Attorney Stephen Zappala Jr. said he’s using an unspecified “different mechanism” to probe Thompkins’ shooting because police haven’t fully cooperated.

Last week, Northampton County District Attorney John Morganelli announced a grand jury investigation into whether state police should investigate shootings involving their own troopers. The investigation was prompted by a dispute between Morganelli and state police, following the fatal shooting of a suicidal man by troopers on May 20.

Police “unilaterally changed their relationship with my office” following the Thompkins shooting, said Zappala.

“I’m not satisfied we were able to get on scene in a timely fashion and talk to people who could give us evidence,” he said.

City Public Safety Director Wendell Hissrich denied that, saying he was at the scene with police Chief Scott Schubert when Zappala’s investigators arrived.

“We assisted them and provided any guidance and assistance that they requested at the scene,” Hissrich said.

Police haven’t publicly named the officers involved in the incident or said which of them shot Thompkins, but have said that the officers returned fire after Thompkins shot at the fleeing intruder, toward the responding officers.

Although Zappala’s investigators have talked to witnesses, he said interviews conducted with the officers involved didn’t occur the same day as the shooting, so they could have had time to review other evidence, or talk with legal experts before speaking with the DA’s staff.

“Once you start looking at video, stuff like that, you sit down with the lawyers — your testimony becomes less viable,” Zappala said.

The officers involved in the shooting were on administrative leave for 10 days after the incident, before they returned to duty.

Hissrich wouldn’t say whether Zappala’s investigators were kept from interviewing the officers the day of the shooting. But he said city policies were followed and haven’t been recently changed.

Hissrich also said there’s been “ongoing dialogue” about improving the city police relationship with Zappala’s office.