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The gun was not connected to the Wilkinsburg killings, Mr. Jubas said.

The case stunned the county and Wilkinsburg, a suburb just east of Pittsburgh with a population of about 16,000 that has struggled with drug trafficking and gun violence.

Investigators in the Wilkinsburg case, Mr. Jubas said, based charges against Mr. Thomas and Mr. Shelton on a “very poorly cobbled together investigation.”

One night, at a barbecue in Wilkinsburg, Pa., near Pittsburgh in 2016, a gunman fired at the partygoers, driving them to a house’s back porch, where a second gunman picked off victims with an assault-style rifle.

Before Williams became a Pittsburgh officer, his encounters with police had included running from them as a child in Wilkinsburg, a mostly Black borough in the Pittsburgh metropolitan area.