(CNN) -- Authorities now believe Haleigh Cummings is dead and are treating the 5-year-old's disappearance as a homicide investigation, the Putnam County, Florida, sheriff said Thursday.
"At this point, I'm comfortable in saying I'm calling it a homicide," Sheriff Jeff Hardy said. He added that investigators are looking at "one or more persons of interest" but would not identify any targets of the investigation.
The sheriff spoke with reporters as divers completed their search of the St. Johns River, south of Satsuma, Florida, where Haleigh lived in a mobile home with her father, Ronald Cummings, and his then-girlfriend, Misty Croslin.
Hardy said the searched was prompted by "leads" developed when people he would not identify began cooperating with investigators.
"People are starting to talk," he said. "We're hoping to make an arrest in this case," he said.
Holding up a photograph of Haleigh, the sheriff said: "We are trying to bring someone to justice for this little girl right here."
He later vowed: "I am not going to give up on this little girl and her family."
Croslin, who is in custody on unrelated drug charges, was spotted on Wednesday on a dock near a boat launch where the search was centered, according to a helicopter videographer with CNN affiliate WKMG-TV. The woman was handcuffed and spoke with investigators, who were pointing at the murky river.
Croslin was the last person to see Haleigh before she was reported missing on February 9, 2009.
"I think she's come to realize she's in trouble," Hardy said.
About 25 to 30 divers searched the river for three days, the sheriff said. The searched the river by hand, in zero visibility, Hardy added, describing the conditions as difficult.
"We found everything from alligator toenails to, you know, you name it," Hardy said.
The search of the river would "wrap up" at about 6 p.m. Thursday, the sheriff said. Divers have several items from the river, he said, but would not be more specific. He said the items would be sent to a lab for analysis.
Croslin told police she tucked Haleigh and the girl's 4-year-old brother into bed about 8 p.m. the previous night and went to sleep two hours later.
She said she awoke at 3 a.m. to find Haleigh missing and a cinder block propping open a back door. Ronald Cummings reported Haleigh missing when he returned from work at dawn.
An attorney representing Joe Overstreet, Croslin's cousin, said Florida officials questioned his client Tuesday.
Overstreet, who is from Nashville, Tennessee, was in the Satsuma area when Haleigh disappeared, said attorney Shawn Sirgo. Florida Department of Law Enforcement agents flew to Nashville on Tuesday to question him, Sirgo said.
Overstreet maintains he had nothing to do with Haleigh's disappearance, Sirgo said, adding that his client is "broken over this" and "knows he is being used by people whose story keeps changing."
Putnam County authorities said in August that "the evidence and investigatory effort has minimized the likelihood that Haleigh's disappearance is the work of a stranger." Police have said Cummings and Crystal Sheffield, the girl's mother, are not considered suspects.
Jail records show that Croslin was removed from the St. Johns County Jail, where she is being held on suspicion of drug trafficking, on Tuesday, CNN affiliate WJXT has reported. The sheriff told the station she was brought to Putnam County for a questioning.
Investigators have said they believe Croslin "continues to hold important answers in the case" but has not provided "any sort of detailed accounting of the hours during the late evening and early morning of Haleigh's disappearance." Authorities have previously said physical evidence contradicts Croslin's account of the events that night.
However, police have stopped short of naming Croslin as a suspect. In televised interviews, Croslin has said she does not know what happened to Haleigh but believes "the other side of the family" knows where she is.
Croslin and Cummings were both arrested in January along with three others after allegedly selling about $3,900 worth of drugs to undercover officers, authorities said. The two divorced following Haleigh's disappearance.
HLN's Natisha Lance and Jean Casarez contributed to this report.