A Florida judge on Friday denied a request from a former police officer who fatally shot a driver to have criminal charges against him thrown out under the state’s so-called Stand Your Ground law, calling his testimony “unreliable and not credible.”

The defendant, Nouman K. Raja, was on duty in plain clothes early on Oct. 18, 2015, when he approached a vehicle that he said he thought was abandoned on the side of an interstate. His brief exchange with the man he found inside the vehicle, Corey Jones, was recorded on a phone call Mr. Jones had placed seeking roadside assistance.

At the time of the deadly encounter, Mr. Jones, a 31-year-old musician and housing inspector, had a legally purchased handgun with him. Mr. Raja has claimed that Mr. Jones, who was black, pointed it at him, but prosecutors say Mr. Raja fired shots even as Mr. Jones fled. Within moments of making his approach, Mr. Raja had fired six shots and struck Mr. Jones three times, killing him.

Less than a month later, Mr. Raja was fired from the Palm Beach Gardens Police Department. He was charged in June 2016 with manslaughter by culpable negligence and attempted first-degree murder with a firearm.