This story was supported with funds from Spotlight DC—Capital City Fund for Investigative Journalism.

No prosecutor likes to lose. But for Michael Ambrosino, one adverse ruling in September 2019 probably stung more than most.

Back then, Ambrosino had a leading role at the U.S. Attorney’s Office for D.C, where he managed the prosecution of cases involving DNA and other forensic evidence. And some of that evidence was a key part of the government’s case against Marquette Tibbs, who federal prosecutors believed had shot and killed a business associate during a scuffle in Anacostia in November 2016.

Analysts at D.C.’s crime lab, formally known as the Department of Forensic Sciences, had examined a gun that police claimed Tibbs threw away as he fled the scene of the shooting and concluded that it matched shell casings found there. Ambrosino hoped to call one of those firearms examiners, independent contractor Chris Coleman, to testify to those findings at trial, but Tibbs’ lawyers objected.

Ambrosino and attorneys at the Public Defender Service of D.C. squared off in a hearing before D.C. Superior Court Associate Judge Todd Edelman to argue over the matter, but neither side expected much drama: Judges typically allow such evidence at trial except in rare circumstances. But Edelman surprised them and opted to severely curtail what Coleman could testify about in court,…