Police say a man approached a woman from behind at a public library near Boston and stabbed her repeatedly with a hunting knife, killing her and injuring an elderly man who came to her aid.

Jeffrey Yao (Winchester Police Department via AP)

Middlesex District Attorney Marian Ryan says the 22-year-old woman was seated at one of Winchester Public Library's reading rooms Saturday morning when 23-year-old Jeffrey Yao stabbed her with a 10-inch knife.

Police say there is no motive determined yet.

Ryan said the woman headed toward the door and a number of people came to her aid, including a 77-year-old man who was also stabbed. Ryan said the man suffered non-life threatening injuries.

Yao is charged with murder and will appear in court Monday. A phone number for him could not be found.