There was no indication of forced entry when police, responding to a 911 call, arrived at her Fifth Avenue apartment.
An autopsy revealed Stein died from "blunt impact injuries of the head and neck," said Grace Brugess, a spokeswoman for the medical examiner's office.
Police are treating Stein's death as a homicide, but have refused to identify any possible perpetrators, CNN said.
"She had a tough exterior but she had a heart of gold," said Dottie Herman, chief executive officer of Prudential Douglas Elliman, the firm for which Stein worked.
Stein had been battling breast cancer and "was a little wobbly on her feet some days," Herman said. "I just assumed Linda fell and hit her head when I first heard about this, because I can't imagine anyone that would want to kill Linda."