Published: Oct 12, 2010 5:36 PM EDT
Updated: Oct 12, 2010 2:03 PM EDT

HICKORY, N.C. (AP) - Police said Tuesday they are shifting to a homicide investigation in the case of a missing 10-year-old North Carolina girl whose bone cancer left her with a prosthetic leg and hearing aids.

Hickory Police Chief Tom Adkins said at a news conference that the girl's stepmother confessed to writing a ransom note found at the house where the girl lived, and she has been charged with felony obstruction of justice in the case. Elisa Baker, who was already being held on unrelated charges, has also asked for an attorney.

The girl, Zahra Clare Baker, was reported missing Saturday by her father, but authorities have cast doubt on what he and the stepmother told them. Adkins said investigators can't find anyone outside the immediate family who has seen the girl in the last month.

Uncertainty over when she was last alive has stymied efforts to enlist volunteers to search for the girl, the chief said.

"We understand the public wants to help find Zahra. The problem is we cannot confirm with any confidence how long Zahra has been missing," he said. "Without this information, we cannot positively select the area to search for her."

Adkins did not take any questions and did not offer further explanation of why the case was being treated as a homicide. A search warrant revealed on Monday that police dogs detected the smell of human remains on cars belonging to the father and stepmother.

The father, Adam Baker, has not been charged in the case.