OAKLAND — An ex-convict will not be charged with murder for a Jan. 18 killing police believe sparked a retaliatory attack last week that left two people dead and three wounded, authorities said Monday.

Police said prosecutors have determined Derrick "Dirty D" Knockum, 39, acted in self-defense when he fatally shot Purnell Brewer, 26, of Berkeley in the 2000 block of East 26th Street.

Knockum was charged with being a convicted felon in possession of a gun and violating parole.

On Monday, witnesses to the shooting confirmed that both Knockum and Brewer, who were longtime friends, had guns, homicide Sgt. Ersie Joyner III said.

Joyner said Knockum "had a legitimate fear he might be shot" and defended himself.

The dispute started because Knockum was angry with Brewer over the way Brewer drove up to the East 26th Street site, an area where Brewer sold drugs, Joyner said.

Police believe that killing prompted a drive-by shooting about 8:40 p.m. Thursday in the 2700 block of 22nd Avenue near a recreation center, where Knockum occasionally hung out. Two people died, and three were wounded.

Killed was Knockum's 19-year-old nephew, Willie Clay, and Knockum's close friend, Willie Jason Williams, 31, both of Oakland.

Police said the gunman probably did not know that about the time he opened fire on the group, Knockum, who is on parole for robbery, was admitting his involvement in Brewer's death to Joyner and Sgt. Brian Medeiros

at police headquarters.

Police and Crime Stoppers of Oakland are offering up to $35,000 in reward money for information leading to the arrest of the gunman in the Thursday shooting.

Anyone with information is asked to call police at 238-3821 or Crime Stoppers at 238-6946.