The city’s top forensic pathologist told a federal jury in Brooklyn yesterday how gunpowder residue found on the bodies of two undercover detectives offers clues about the moment of their executions.

The pathologist, Charles Hirsch, delivered his testimony in the capital trial of Ronell Wilson, who is accused of killing detectives James Nemorin and Rodney Andrews in Staten Island in 2003, which is in its third week. An accomplice of Mr. Wilson’s, Jessie Jacobus, has testified that Mr. Wilson shot the two detectives from the backseat of a car as they sat up front. Yesterday, Dr. Hirsch’s testimony confirmed aspects of what Jacobus had said about how the murder occurred.

Dr. Hirsch is the first witness to suggest that Nemorin put up a struggle against his killer in the Nissan Maxima. His theory originates from the fine black dust present on Nemorin’s left hand, which Dr. Hirsch said was likely caused from the burned gunpowder of a revolver shot.

“In my opinion, he had to be reaching over his head, as if, for example, he had been trying to reach for the gun,” Dr. Hirsch, the city’s chief medical examiner, said.

Dr. Hirsch began his nearly two hours of testimony by describing the dust and specks that spent gunpowder can leave on victims of gunshots fired at extremely close range. This grim lecture was followed with photographs of the two fatal gunshot wounds. From these photographs, Dr. Hirsch attempted to reconstruct the relative position of the shooter to the two detectives.

The jurors, whose identities have not been disclosed, appeared to follow Dr. Hirsch’s testimony with interest, with several members leaning forward in their chairs for a better view of the autopsy photographs.

The defendant reacted differently. At one point, Mr. Wilson, 24, began to rub his temples, a rare show of expression.

Mr. Wilson is charged under a federal racketeering indictment that could carry the death penalty if he is convicted. Nemorin and Andrews, part of the NYPD’s elite Firearms Investigations Unit, had been attempting to purchase a gun from Mr. Wilson and Jacobus. Instead of selling the detectives a firearm, Mr. Wilson had decided to murder and rob the detectives, prosecutors have said.

Prosecutors have said Andrews was shot first and Nemorin shortly after, although Dr. Hirsch said he did not have an opinion on the relative timing of the murders.

Dr. Hirsch did not perform the autopsies on the two detectives, a fact Mr. Wilson’s attorney seemed to underscore when she several times mistakenly addressed Dr. Hirsch by the name of the pathologist who did perform the autopsy.

The attorney, Kelly Sharkey, questioned why the black dust on Nemorin’s hand was not mentioned in the original autopsy report.

Dr. Hirsch described it as “an oversight.”

Ms. Sharkey suggested that the dust could have come not from gunpowder but from the street in Staten Island, where Nemorin’s body was dumped. Dr. Hirsch rejected that theory, saying the “little groove” between the thumb and index finger was a “funny place for dirt from the street” to end up.