John Nicosia was inside the Emergency Medical Service station in Astoria, Queens, where he worked when a frantic man ran up to report a stabbing.

Mr. Nicosia, an emergency medical technician, rushed to the corner of 41st Street and 20th Avenue, where a figure lay on the sidewalk. Mr. Nicosia immediately recognized Alison Russo, a lieutenant whom he had seen inside the station just minutes earlier.

“She wasn’t breathing on her own, no pulse,” Mr. Nicosia testified, adding: “I immediately started doing what I was trained to do.”

But the CPR that Mr. Nicosia performed that afternoon in September 2022 could not preserve his colleague’s life, and on Monday jurors in State Supreme Court in Queens found Peter Zisopoulos guilty of murdering her.

Prosecutors presented a wealth of evidence tying Mr. Zisopoulos to the murder — crucially including video of the killing in progress. But Justice Ushir Pandit-Durant stopped the defense from introducing evidence that Mr. Zisopoulos, whose mother told authorities he took medication for schizophrenia, was mentally ill. The judge said that defense lawyers had not filed a notice that would have let prosecutors prepare a rebuttal.

During a summation, one of Mr. Zisopoulos’s lawyers suggested to jurors that his client was not “astute” or “aware” and lacked the ability to form the intent to kill, an element of the murder charge he faced.