ST. LOUIS -- In what police say may have been a mercy killing, a licensed practical nurse is accused of suffocating a 91-year-old resident of a St. Louis County nursing home.

The nurse, Donna Hohl, 51, of St. Louis, was charged Wednesday with second-degree murder. Officials said that Hohl, being held on $250,000 bail, admitted to detectives that she killed Helen Schreiber at the Bethesda Southgate Nursing Home on May 1, the St. Louis Post-Dispatch reported.

Schreiber was presumed to have died of natural causes until later in May, when police got a tip from the Missouri Department of Aging. There had been no autopsy and officers declined to discuss how state officials became aware of the case.

Investigators exhumed Schreiber's body June 17 and found evidence that she had been murdered.

"It appeared that she was close to natural death at the time," said Lt. Jon Belmar of the St. Louis County police.

Kim Bass, a spokesman for the Bethesda Health Group, said nursing home officials immediately suspended Hohl when police notified them of the investigation. She was terminated two weeks ago, Bass said.

Police remain uncertain about a motive in the case. They said Hohl did not benefit financially from Schreiber's death and may have wanted to end the woman's suffering.

Schreiber had lived in the independent living center of the nursing home until a month before she died, when she moved to the skilled nursing unit.

She had no immediate health problems other than those associated with old age, officials said.