A trail of blood led from one apartment to another. That was the first clue that led the police to suspect a 24-year-old woman in the fatal stabbing of her neighbor in an Upper West Side co-op.

Detectives made inquiries. The young woman, who lived with her mother, had been hospitalized for mental illness. A search of her apartment turned up other evidence, the police said, including blood on a shoe.

On Wednesday, more than a week later, the police arrested the woman, Anya Johnston, on charges she slashed the throat of Susan Trott, a 70-year-old advertising copywriter who lived one floor below her, the police said.

Ms. Johnston was charged with murder and burglary. The police said she was emotionally disturbed. The week of the murder, Ms. Johnston had been reported missing by her mother, but eventually had returned home, the police said. She told her family that she was mentally unstable and was taken to Mount Sinai St. Luke’s hospital on the day of the murder, the police said. She had undergone a psychiatric evaluation before she was taken into custody on Wednesday.