Further toxicology tests and investigation are needed to determine how Celina Cass died, Senior Assistant Attorney General Jane Young said at a news conference.
"Despite the findings, the attorney general's office continues to investigate her death as suspicious," Young said. "A criminal investigation remains ongoing into the circumstances of her death."
New Hampshire Fish and Game Department divers found Celina's body Monday near a hydroelectric dam that spans the Connecticut River between Celina's hometown, Stewartstown, and Canaan, Vt., ending an intense search that at its peak involved more than 100 federal, state and local law enforcement officers.
Young said "visible observations" of Celina's body in the water and once it was removed prompted investigators to consider her death suspicious, but she declined to be more specific. She also declined to comment on whether there were any suspects in the death or say who was in the girl's house the night before she was reported missing July 25. Police have said there was no sign of a struggle and there was no indications she ran away or someone took her.
Community residents were saddened that the autopsy wasn't able to provide them with any closure.
"I don't even know what to say. I thought the community needed some answers, and I thought we were going to get them tonight," said Debbie Whelan, whose daughter, a friend of Celina's, was sobbing after watching a news report on a TV monitor.
The lack of answers was "scary," said Shannon Towle, who owns a convenience store across the street from the house where Celina lived with her mother, sister, stepfather and a man named Kevin Mullaney, said by neighbors to be the son of a former boyfriend of her mother, Louisa Noyes.
"We still don't know what happened, and that's terrifying," Towle said.
After the news conference, teenagers gathered in a town park, where candles were lit in Celina's memory.
"Someone has to light the way for Celina," said Kayla Baglio, 18. "It's to let her know no matter what, people are going to be here for her."
Earlier Tuesday, investigators combed the area along the river, which divides Vermont and New Hampshire, above where Celina's body was found. A crime scene technician said they were doing computer-aided diagramming to give them a precise electronic image of the area.
Celina's stepfather, Wendell Noyes, who reportedly was hospitalized hours before her body was recovered from the river, returned to the community Tuesday.
Witnesses said Noyes was taken away by ambulance Monday after lying down with his face in his arms outside a home where he had been staying in Stewartstown, a community of 800 residents. Hospital officials declined to say whether he was a patient.
Towle, owner of Towle's Mini-Mart, said she sold him cigarettes Tuesday morning.
In the first days after Celina disappeared, Noyes had described her as a quiet girl who would not have left home on her own, but he had declined to comment further.
Residents considered canceling their annual children's fair in light of the fifth-grader's death but instead decided to go forward with the weekend event and to dedicate it to her memory.
"It's still a scary place for our children," Patricia Grover, who's on the organizing committee for Stewartstown Days, told The Associated Press. "They need something that's on a little happier note for them."
Associated Press writer Holly Ramer contributed to this report from Concord.