The tearful foster father of a missing 5-year-old boy with cerebral palsy said Thursday he believes the child was kidnapped and pleaded for his safe return.

Meanwhile, authorities intensified their round-the-clock search for Hasanni Campbell, who disappeared Monday in an upscale Oakland neighborhood.

His foster father, Louis Ross, the last known person to see Hasanni, said the boy disappeared after he briefly left him outside his car in the rear parking lot of a shoe store where Ross' fiancee, Jennifer Campbell, works.

Ross said he went to the store's front entrance to ask Campbell to open the back door, but when he returned to the parking lot, Hasanni was gone.

"We refuse to give up hope. We know he wouldn't walk away like that," Ross told The Associated Press Thursday. "Whoever has him, let him go. Please drop him off somewhere — the police, a hospital, a school. Somewhere. Please."

As many as 70 officers from several law enforcement agencies, including the FBI, have joined the search for Hasanni. They scoured parts of Oakland and searched numerous areas, including nearby parkland and his foster parents' home in Fremont.

"It has been nonstop," Oakland police Sgt. Ray Backman said. "Given that it's a 5-year-old boy, time is a critical factor for us."

The boy is still considered a missing person, not a kidnap victim, Backman said. Investigators were working with no significant clues and a limited amount of tips, he said.

"There's always that possibility he will be found," the sergeant said.

Ross said his family was cooperating with authorities, although he felt police believed he and Campbell did something wrong.

Ross has taken a polygraph test, but Campbell declined because she is six months pregnant and worried about any adverse effects.

Ross declined to discuss the results of his test.

The couple, who also have custody of Hasanni's year-old sister, were seeking advice from John Burris, a civil rights attorney.

Burris said they were surprised about being the focus of the investigation. Their house was searched and Hasanni's sister was taken from them and placed in protective custody, he said.

"What I told them is their boy is missing and that you have to cooperate, assuming that you did nothing wrong," Burris said. "I told them, you were the last ones who saw him, so they are going to come to you."

Backman said he understood the family was going through a trying time.

"A lot of the questions that we have to ask during the course of investigation are very intrusive because we want to explore all possibilities, however remote they might be," Backman said. "We're trying to be as delicate as we can."

Ross said Hasanni wears arch-support braces to walk — not full leg braces as previously reported — because of his cerebral palsy.

Police said the boy is black, 3 feet tall, weighs 30 pounds and was last seen wearing a gray sweat shirt and gray pants.

He disappeared after Ross pulled his BMW into a rear parking lot behind the shoe store with Hasanni and his infant sister onboard. Ross said he was heading for an orientation at Stanford University Medical Center in Palo Alto and then to his twice-weekly medical assistant class in Fremont.

Dropping the kids with Campbell is a family routine, Ross said. When he opened the rear passenger door for Hasanni, he said, the boy was unbuckling his seat belt.

"I said, 'Hasanni, go wait by the back door,' and he had already taken a first step out of the car," Ross said.

Ross said he grabbed Hasanni's sister and went to the front of the store and told Campbell to open the back door. He then went back to the parking lot.

"When I got to back there (Campbell) was already there. She says to me, 'Where's Hasanni?"' Ross said. "I said, 'What do you mean?' I look to see if he's standing along by the car, but he's not."

Ross said he and Campbell asked her co-worker if he had seen Hasanni. Ross then ran back to his car and called 911.

Now, he and his family wait, worry and plead for Hasanni's safe return.

"We just want him back home with us." Ross said. "Please."