Celina Cass' biological father, Adam Laro, told a Boston news station that he felt Wendell Noyes, Celina's stepfather, was behaving strangely in the days following the 11-year-old's disappearance.
"Thing [sic] started to become different ... something was weird -- something was strange but I couldn't place it. Something was strange," Laro told WHDH-TV.
Celina Cass vanished from her West Stewartstown, N.H., home on July 25. Her body was pulled from the Connecticut River one week later.
Investigators are treating the death as suspicious.
Cass was last seen on the evening of July 25 in her bedroom. Local New Hampshire news station WMUR reported that her mother and stepfather told authorities the girl was gone when they went to wake her up the following morning.
On Monday morning, Celina's stepfather Wendell Noyes was taken to the hospital via ambulance, after behaving bizarrely outside his duplex home. He has since been released.
WMUR reported that Wendell Noyes was seen through the window of a West Stewartstown, N.H., home on Tuesday, sitting at a table eating a sandwich. He waved to reporters but refused to come out and speak.
In 2003, Noyes was committed to a hospital with a diagnosis of schizophrenia, ABC News reported. ABC obtained court records showing that Noyes has an arrest record for violating a restraining order and threatening his ex-girlfriend.
An autopsy was performed on Celina's body on Tuesday, but the results were inconclusive.
Residents of the northern New Hampshire hamlet are anxious to know how the shy girl died.
Assistant Attorney General Jane Young said in a news conference Tuesday evening that toxicology tests are expected to shed more light.
"We are doing everything we can to have more definitive answers,'' The Boston Globe quoted her as saying.
Young did not comment on whether any suspects had been identified, and encouraged West Stewartstown residents to be vigilant about safety, but discouraged an atmoshere of "heightened alarm.''
"At night, I'm afraid that the person who did this will be out there and that they'll come into our house and take me and my brother,'' Bailey Swallow, 12, told the Boston Globe.
Garrett Rancourt, 10, told the newspaper he sleeps with "one eye open."
Online commenters to a story about the girl published Tuesday on WMUR's Web site expressed concern about Celina's stepfather.
Several comments call attention to his Facebook profile, referring to Noyes as a "creeper" and questioning whether the Cass family was aware of the profile.
The overwhelming majority of Noyes' Facebook "friends" are young women from different parts of the country, some appearing to be amateur models. The page -- which is available for the public to view -- shows that Noyes posted flirtatious comments on some of the girls' profiles as recently as June.
Celina's paternal grandmother, Marica Laro, spoke about Celina's death in the WHDH-TV interview.
"All I can say is unbelievable -- just unbelievable," she said. "We have a lot of questions -- we have our own suspicions but we can't say anything."
In a CNN interview on Tuesday, Adam Laro said he could not understand why anyone would want to hurt his daughter.
"She was very kind in spirit," Laro said. "She, I guess you could say, was an everyday child who loved being in life.''
Laro added that Celina "did not seem suspicious or uncomfortable" when they last met and discussed her living situation, the Boston Globe reported.
On Tuesday, investigators went back to the home Celina shared with her mother, stepfather and sister to continue a search.
"Now because we are treating this as a suspicious death, we are going back to see if there's any evidence that could assist in our criminal investigation," Jane Young told the Globe in a phone interview.
Authorities seized a Ford F150 pickup truck with New Hampshire license plates from the home.