Hasanni Campbell, who lives in Fremont with his foster parents, Louis Ross and Jennifer Campbell, was reported missing from the parking lot of the Shuz of Rockridge shoe store in the 6000 block of College Avenue in Oakland about 4:15 p.m. nine days ago.
Thomason said police talk to Hasanni's foster parents every day and that they are being cooperative.
Ross has been questioned several times by police and has taken a lie detector test.
On Monday, he answered speculation that he might be involved in Hasanni's disappearance. He told reporters to talk to social services about the placement of Hasanni in his home.
“You go to them and ask them why they placed them with us. Because we gave a damn! These were our children. Don't try to take that from us. This wasn't a situation where we didn't care about those children. We've fought every inch for both of them,." Ross said.
The couple was also serving as foster parents to Hasanni's younger sister. That child has been taken from the couple.
A reporter asked Hasanni's grandmother if she thought Ross had anything to do with the disappearance and she said no and adding that she trusted Ross completely.
Thomason said police are still treating Hasanni's disappearance as a missing persons case and that there is no evidence a crime has been committed.
"We're asking the public to call if they have any credible tips," Thomason said.
The child's relatives held a vigil on Monday afternoon in Rockridge. Only about two dozen people turned out.
Police said they have followed up on all of the approximately 50 tips they've received so far but none have panned out. Thomason has told reporters that 50 is a very low number when it comes to high profile cases like Hasanni's. They normally come in by the hundreds.
Oakland police and Crime Stoppers is offering a $10,000 reward for information leading to the whereabouts of Hasanni.
Thomason said one caller left about 40 unhelpful messages with Crime Stoppers and clogged its voicemail box. But the voicemail box is now open again, he said.
On Saturday, law enforcement officials and volunteers searched marshland and shoreline areas at the tip of West Winton Avenue in Hayward as well as Coyote Hills Regional Park in Fremont but didn't find anything significant.
Thomason said police have also searched North Oakland thoroughly.
Police describe Hasanni as a black boy with medium complexion, brown hair and brown eyes. He is about 3 feet tall and weighs about 30 pounds. He was last seen wearing a gray sweatshirt and grant pants.
Police said he has some difficulty walking because he has cerebal palsy.
Thomason said people with information on the case should call the Oakland Police Department or Crime Stoppers at 510-777-8572 or 510-777-3211.
Bay City News contributed to this report.