A 5-year-old boy was at the window of his Bronx apartment, screaming frantically for help. He and his 2-year-old sister had been alone in the apartment for hours after finding their mother lifeless on the bedroom floor.

A passer-by answered the boy’s panicked cries and called 911, according to a law enforcement official. When officers arrived, the police said, they discovered that the mother, Luz Perez, was dead and that she had injuries to her head and body.

On Tuesday, more than two months after the Oct. 13 incident, the police arrested a man with a previous manslaughter conviction who was free on parole, accusing him of strangling Ms. Perez.

The man, Asun Thomas, 46, was charged with murder, the police said. He was ordered held without bail and sent to the Vernon C. Bain Center, the floating jail in the Bronx, court and city correction department records show.