The Doors on the 13th Floor

Elderly Agatha Leighton returns from a trip to find her apartment on the thirteenth floor has been robbed. As she attempts to call the police, someone grabs her. Sally, who lives down the hall, attempts to visit the old lady as she'd promised, but finds she's been locked in her apartment and the phone is dead. Through the door's peephole, she sees the building's desk clerk leaving Agatha's apartment. Sally realizes something bad has happened to the old lady.