The most recent incident occurred around 5 p.m. Monday, when a 9-year-old girl was walking in the 3300 block of Crestmoor Drive, police said.
A white cargo van with a red ladder on the top pulled up next to the girl, and the driver, described as a clean-shaven white man in his 30s with brown hair and wearing a green T-shirt, asked the girl to get into the car.
The girl ran away, and the van drove north on Crestmoor Drive and disappeared, police said.
Officers are also investigating a similar incident that occurred Saturday in the area of Crestmoor Drive about a half-mile from Monday's incident.
A group of boys was playing in the front yard of one of their homes in the 2500 block of Rosewood Drive around 7:35 p.m. when a newer-model white two-door Pontiac with red interior drove up next to them, police said.
The driver, a white man in his 30s with blond, shoulder-length hair and wearing a gray sweatshirt, asked the boys if they had lost a ball, according to police.
The boys told their parents, and the vehicle was last seen headed north on Crestmoor.
Police Cmdr. Noreen Hanlon said today that the Crestmoor area is a "notoriously" quiet, nice neighborhood.
She added that police are considering the possibility that the driver was the same man in both cases and are working to develop a sketch of the suspect.
"The vehicles are definitely different, but the suspect description is similar in both instances," Hanlon said.
Hanlon said the mother of one of the boys targeted in Saturday's incident believes she has seen the white Pontiac in the neighborhood over the past several weeks.
Hanlon said that anyone who sees vehicles at all similar to the vehicles described in these cases is urged to contact police at (650) 616-7100.
"It's important that we find out who this individual is for obvious reasons," Hanlon said.