The FBI has joined the search for a missing Oakland, Calif., boy with cerebral palsy, which has turned to a nearby park.

FBI agents joined local police Wednesday in a search of Nordvik Park at about 8 a.m. local time. They did not say why they were in the park or what led them there, KTVU.com reported.

Hassani Campbell, 5, was last seen Monday afternoon sitting in the car parked outside a shoe store where his aunt works. She has custody of the boy.

The man watching the boy at the time told authorities that he left the car to unlock the store door to give the boy, who wears braces on his legs, easier access. He said the boy was gone when he returned.

Police initially described the man as Hassani's father but later said he was a "caretaker."

Police describe the boy as black, standing 3-feet-tall and weighing 30 pounds. He is wearing a gray sweatshirt and gray pants.

Anyone with information about the boy's whereabouts is asked to call the Oakland police youth services division.

Click here for more on this story from KTVU.com.

The Associated Press contributed to this report.