York, Pa. ? A former mayor and two other white men went on trial for murder Tuesday, with a prosecutor saying all three played a role in the ambush slaying of a young black woman during a 1969 race riot.

Standing trial are Charlie Robertson, a policeman at the time of the riots who went on to become mayor, and two men who prosecutors say were members of white gangs that attacked the car carrying Lillie Belle Allen, a preacher’s daughter visiting from Aiken, S.C.

Allen, a 27-year-old seamstress, was gunned down by a mob, her body nearly cut in half. Her case was reopened in 1999, along with that of a slain patrolman, the only other person killed in the rioting.

According to prosecutor Thomas Kelley, police had struck an alliance with white gang members, eager to get revenge for the shooting of the patrolman on the second day of rioting.

Three days later, Allen and four family members made the deadly mistake of turning their white Cadillac into a white neighborhood where armed gang members had gathered, Kelley said.

Defense attorneys argued that the 96-member police force was overwhelmed and had advised residents to protect themselves. They also said the neighborhood where Allen was killed had been shot at by occupants of a white Cadillac twice in two preceding days.

The other defendants are Gregory Neff, who told a grand jury he fired at Allen’s vehicle three times, and Robert Messersmith, who allegedly fired the shotgun slug that killed Allen.

Robertson has acknowledged that he shouted “white power” at a rally to unify members of warring white gangs the night before Allen was shot. He has denied the other accusations against him that he incited whites to violence against blacks and provided ammunition to at least one of the shooters.