A bystander who was fatally shot on a Brooklyn stoop may have been killed by a police officer’s errant round, ballistics testing indicated on Wednesday.

The 9-millimeter handgun that investigators say one man used to kill another on a street in Crown Heights on Monday has been eliminated as the source of a bullet that fatally struck the bystander, Denise Gay, 56, a retired home health aide, the police said.

After the violence on Monday, some witnesses said they thought the man who was killed, Eusi Johnson, also had a gun, but none has been found.

Asked if a bullet fired by a police officer could have been the fatal shot, Paul J. Browne, the Police Department’s chief spokesman, said it could have been. “Was it one of the officers’ Glocks? Possibly,” he said. “Was it from the gun witnesses said Mr. Johnson had, but has not been recovered? Possibly.”