The death of an 11-year-old girl whose body was found in a river last month was a homicide, New Hampshire authorities said Thursday.

Investigators said they also know what caused the death of Celina Cass, but they aren't making it public.They say doing so could harm the integrity of the investigation.

"The investigation into the facts and circumstances of Celina Cass's murder remains active," Senior Assistant Attorney General Jane Young said in a statement.

Celina was reported missing July 26. Her body was found a week later in the Connecticut River, not far from her West Stewartstown home.

A reward has been offered for information leading to an arrest. No suspects have been named.

Young said Thursday that the cause of death was determined through information gleaned from the investigation and from toxicology tests.

The news came after investigators met with Celina's mother and sister for two hours. Luisia Cass told The Associated Press she had no comment after leaving the Coos County attorney's office in Lancaster.

Luisia Cass was accompanied Thursday by a friend and her 13-year-old daughter, who was clutching a stuffed panda bear.

A family friend confirmed that Luisia Cass has separated from Celina's stepfather, Wendell Noyes. He has a history of psychiatric issues and has been in and out of hospitals since Celina disappeared.