PETALUMA (CBS SF/BCN) -- A Good Samaritan was robbed at gunpoint in Petaluma Sunday night after stopping to assist a man who flagged him down near Casa Grande High School, authorities said.

Petaluma police said they received a 911 call at around 8:17 p.m. from a victim who said they had just been robbed at gunpoint.

According to investigators, the victim said he had been flagged down by an unknown male at the entrance of the school, asking for help. He stopped to offer assistance, but when rolled down his passenger side window, the suspect pointed a gun at him, demanding his money and phone.

The suspect grabbed the victim's wallet before fleeing. The victim, meanwhile, drove to a safe location to make the 911 call.

The suspect was described as a white man in his 20s with dark hair, wearing a flannel shirt. The gun was described as a black, semi-automatic handgun.

Anyone with information about the case is asked to contact the Petaluma Police Department at (707) 778-4372, or utilize ppdtips@cityofpetaluma.org