Police bloodhounds were unable to detect the scent of a missing 5-year-old boy outside an upscale shoe store where his foster father says he last saw him, according to a police search warrant affidavit.

Police were mystified as to how Hasanni Campbell could have vanished Aug. 10 from a "crowded business district with no witnesses," said the document, which supported a warrant to search the home of the boy's foster father, Louis Ross, in Fremont.

Ross, the last known person to see Hasanni, said the boy disappeared after he briefly left the boy 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.

Hasanni has cerebral palsy and wears braces on his legs.

According to the affidavit, 10 days before the boy vanished, Ross sent an expletive-filled text message to Hasanni's foster mother, threatening to leave the boy alone on a train station platform, the San Francisco Chronicle reported.

Ross said he would look after Hasanni's 1-year-old sister, but not the boy.

"I will watch her but he will be out on the (station) and its your responsibility," said the July 31 text message quoted by police.

In a phone interview Thursday with the San Francisco Chronicle, Ross said he sent the text message in frustration at a time when he planned to break up with Campbell.

"It was me venting about a situation in our past that had come back up," he told the newspaper. "I was ending the relationship at that point."

Besides scouring the area around the shoe store, police have searched Ross' home, the surrounding neighborhoods and parks as well as a scrap yard in nearby Hayward, Calif., that Ross had visited hours before Hasanni's disappearance.

Nothing was taken from Ross' home, and he voluntarily offered his cell phone to police, court records show.

Also Thursday, Ross told television station KPIX that he failed a lie detector test but has cooperated with police "100 percent."

Oakland police would not comment on the court documents or the lie detector test.

Police spokesman Jeff Thomason says Hasanni's disappearance is still a missing person investigation, but says a homicide investigator is in charge of the case.