Caylee Anthony, a Florida toddler who has been missing since June, is likely dead, the Orange County, Fla., sheriff's department said.
FBI lab tests, "along with additional evidence that has not been made public, leads investigators to the belief there is a strong probability that Caylee [Anthony] is deceased," the department said in a statement.
Authorities have been seaching for Caylee since she was reported missing in July, nearly a month after her disappearance in June.
A law enforcement source confirmed to ABC News last week that air sample tests taken from the car trunk of her mother, Casey Anthony, allegedly revealed evidence of human decomposition. Casey has given the police changing and inaccurate information about the case, according to investigators.
Police last month said hairs the same length and color as Caylee's were found in the trunk of the car, and Casey Anthony's mother, Cindy Anthony, said in a 911 call that the car smelled like there had been a dead body in it.
Cindy Anthony has since retracted those comments, saying the smell could easily have been from garbage in the car.
Over the weekend, Casey Anthony was sent back to jail on new charges, including petty theft and use of a forged check. The $500,000 bail posted by a California bondsman that released Anthony from jail was also rescinded.
At a court hearing that reportedly lasted a little over a minute over the weekend, Judge Mike Miller set Anthony's bail at $3,000 for the new charges.
She is now being held on a total of six charges, including the original charges of child neglect and lying to investigators.
Last week, bounty hunter Leonard Padilla, who helped Anthony secure her bond, also said that, in light of preliminary air sample tests that reportedly indicate a decomposing body was in Anthony's trunk, he now believes that Anthony accidentally killed her daughter.
"I think it was an accident," he said. "Her friends called and said she wouldn't have harmed her, but if she did, she would have freaked."
Padilla, a self-styled celebrity bounty hunter, a few weeks ago offered to bail 22-year-old Anthony out of jail, saying that he thought he could convince her to cooperate in the search for Caylee. Padilla has said Anthony has not cooperated since her release.
Casey Anthony has said her daughter has been missing since mid-June, but she didn't report it until more than a month later. She has not been charged with her daughter's disappearance, though police have said she is a "person of interest" in the case.
She faces charges of child neglect, making false statements and obstructing an investigation for allegedly lying to the police.
Last week, prosecutors released more than 400 pages of documents, including police reports and transcripts of interviews, that portray Anthony as troubled and a habitual liar. The documents also allege that Anthony wanted to give Caylee up for adoption before she was born, but that her mother dissuaded her.
According to the documents, Anthony told police that she left the child with her baby sitter, then couldn't find them when she returned from work. But, no one had lived for months at the address Anthony gave for the babysitter.
Anthony also claimed to work for an Orlando theme park, but investigators say she had not worked there in months. The documents include a poem Anthony wrote on July 7, eight days before she reported her daughter missing: "What is given, Can be taken away. Everyone lies. Everyone dies."