HOBART | Authorities believe 2-year-old Jada Justice likely had been killed and encased in concrete in shallow water for three days in LaPorte County by the time she was falsely reported missing.

Investigators believe they discovered the remains of the missing Portage toddler Thursday near rural Otis, Ind. The remains have not been positively identified as those of Jada, but an autopsy is scheduled for today in Lake County.

MORE: Click here for more reports on the Jada Justice case.

"We feel very comfortable that we do, in fact, have Jada Justice," Lake County Prosecutor Bernard Carter said Thursday. He did not elaborate, calling it an active murder investigation.

Charges are expected to be filed today against Jada's 18-year-old baby-sitter, Engelica Castillo, and Castillo's boyfriend, Tim Tkachik, sources with knowledge of the case said.

Sources familiar with the case said Castillo and Tkachik were arrested Wednesday night at a rural Porter County residence near Valparaiso and were being held Thursday evening in Lake County.

LaPorte County Coroner John Sullivan said his office was called about noon Thursday to an area a couple of hundred yards west of U.S. 421 along Snyder Road just north of the Indiana Toll Road, near Otis.

Officials there found remains that later were transported to Lake County.

The discovery is a grim end to an intense search that reached into its second week and drew countless volunteers who distributed thousands of fliers bearing photos of the missing girl.

Jada's mother, Melissa Swiontek, has said she dropped her daughter off with Castillo on June 8, having planned to travel out of town. Swiontek said she last spoke to her daughter June 12.

The next day, Jada -- placed in a plastic bin in which concrete was poured -- was dumped into a shallow waterway near the small LaPorte County town, according to a source close to the case.

Three days later, on June 16, Castillo reported Jada missing from the Glen Park Gas station in Gary. Soon, the FBI joined the investigation with Gary and Hobart police departments.

Police have cast doubt on Castillo's account of the toddler's disappearance since initial interviews began and as the hunt for Jada gained national attention.

Jada's possible abduction was publicized by CNN, "America's Most Wanted" and through the National Center for Missing and Exploited Children database.

Castillo reportedly had been baby-sitting Jada in her home in the 3900 block of Hobart's Missouri Street the day she reported the girl missing. She told police she left Jada in her unlocked Cadillac about 9:30 p.m. June 16 outside the gas station.

After Castillo went inside to buy milk and cigarettes, she returned to find Jada missing, she said.

Gary police initially held Castillo as a person of interest but later released her without charges.

For more than a week, investigators had concentrated on the wooded areas near Castillo's Missouri Street home. Authorities said Wednesday they had expanded their search but declined to disclose details of the new areas.

It was unclear Thursday if they were still awaiting test results from materials that have been pulled from the home.

Gary police investigators, who also were involved in the case, have since said that Castillo appeared under the influence of a substance when she spoke to police.

Authorities had criticized that Castillo and Tkachik had been uncooperative with the investigation into the girl's disappearance.

A week ago, Hobart police released photos of Castillo and Tkachik to the public in an attempt to elicit tips on the case. Tkachik appeared to have fresh burns to his face, which police have said he had not explained.

A source close to the case said Thursday that the burns were the result of a failed attempt to burn Jada in disposing of her body.

Investigators from the FBI and Hobart police stressed Thursday that no one has been charged in the case and that is an active investigation.

Prior to authorities announcing the possible recovery of Jada's body, Castillo's mother, Rita Rincon, defended her daughter, saying information about Castillo has been misunderstood or misconstrued.

Rincon denied criticism that Castillo had been uncooperative and said Castillo and Tkachik had been out with other volunteers passing out fliers.

Watch footage from Thursday's news conference

View locations in the search for 2-year-old Jada Justice

View Jada Justice is missing in a larger map