Jurors deliberated for about four hours Thursday afternoon and Friday morning before finding Brandon Grimes guilty of first-degree murder in the slaying of Detective Troy L. Chesley. Grimes, 23, was also convicted of two handgun offenses.

Grimes will be sentenced Oct. 21. Prosecutors plan to seek life without parole.

The jury was apparently unmoved by the testimony of Grimes, who said Thursday he was simply waiting for a ride when he heard gunshots and realized he had been hit in the leg.

Prosecutors said that story contradicted nearly all the evidence in the case, including the testimony of two witnesses who placed Grimes at the scene of the crime and identified the murder weapon as one that he had been carrying before the shooting.

Chesley, 34, was shot outside his northwest Baltimore apartment in January 2007. He was not in uniform, and authorities have said it was unlikely that Grimes knew he was a police officer. Police said Grimes was wounded in the leg when Chesley returned fire.

City police disclosed earlier this month that some DNA evidence in the department’s crime lab had been contaminated by DNA from lab employees. Grimes’ trial was the first since that revelation in which a defense attorney attempted to discredit DNA evidence presented by police.

But a crime lab employee testified that the contamination did not compromise DNA evidence taken from the gun and that Grimes “could not be excluded” as the source of some of the DNA.

Grimes is awaiting trial in an unrelated carjacking case.