Lab tests confirmed a decomposing body had been in the trunk of a car driven by a missing girl's mother, and investigators believe the 3-year-old is dead.

Investigators have called Casey Anthony, 22, a "person of interest" in Caylee Anthony's disappearance. She faces charges of child neglect, obstruction and making false statements in the case.

"(FBI) laboratory evidence, along with additional evidence that has not been made public, leads investigators to the belief there is a strong probability that Caylee is deceased," the Orange County Sheriff's Office said. "If any evidence to the contrary is provided, it will be vigorously pursued."

The statement said the forensics tests confirmed there had been a decomposing body in the car. Detectives disclosed at a July hearing the trunk contained hair samples appearing to belong to Caylee, a strong odor and suspicious stain that glowed under black light.

A spokeswoman for Anthony's attorney, Jose Baez, did not immediately return a telephone message Monday. He has repeatedly said his client doesn't know what happened to Caylee.

Caylee was reported missing in mid-July, about a month after she was last seen. Casey Anthony told investigators she dropped the girl off with a baby sitter and went to work. Sheriff's detectives have said those details, and just about everything else Anthony has told them, was a lie.

Anthony was released on $500,000 bail Aug. 21, but returned to custody Friday following new theft charges unrelated to Caylee. Prosecutors have offered Anthony limited immunity in exchange for cooperation, but that was set to expire Tuesday.