Authorities in north Florida canceled an Amber Alert for a missing 21-month-old boy on Monday and say they are now investigating the disappearance of Lonzie Barton as a homicide.

Jacksonville Sheriff's Office Chief Tom Hackney released surveillance video during a news conference that shows the car of the mother's boyfriend, William Ruben Ebron Jr., heading toward where the vehicle was eventually found abandoned and then Ebron running back toward his apartment.

"We're working this as a murder case," Hackney said.

Ebron, who lived with Lonzie's mother, was caring for the boy and his 5-year-old sister when Lonzie disappeared July 24. Ebron initially told police he had left Lonzie in the car with its engine running while Ebron went into the apartment, and the car was gone when he returned, authorities said. The car was quickly recovered, but Lonzie hasn't been located since.

"His whole explanation is nothing but a load of lies," Hackney said.

Investigators have said Ebron isn't cooperating with them. He's in jail without bond on two counts of felony child neglect in relation to the case. Ebron has a lengthy arrest history, records show, including convictions for misdemeanor battery charges, robbery and grand theft. His ex-girlfriend also filed several domestic violence injunctions against him. Jail records didn't list an attorney.

Hackney said he can no longer question Ebron, because Ebron has invoked his Constitutional right not to speak with police. Hackney said he hopes Ebron learns of the surveillance video's existence.

"I can't reach out to him, but what I can do is pass this information on to those who have contact with him," Hackney said. "I want them to pass this on. I want them to let him know what we got."

Hackney said he's not sure what happened to Lonzie, but added he's confident that Ebron has details about the child's fate.

"I want the child's body," Hackney said. "I want the child's body to be taken back to his family, to be put to rest properly. That's the right thing to do."

The sheriff's office has led a massive search of the Jacksonville area since Lonzie's disappearance, leading to the recovery of two unrelated sets of human remains.