NEW YORK (CBSNewYork) — An Upper East Side man was thought to have died of natural causes, but the New York City Medical Examiner’s office ruled Thursday that his death was a homicide.

The body of Christopher Cooley, 78, was found inside his apartment at 64th Street and Madison Avenue on Wednesday, March 16.

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Police sources told CBS2 at the time that people who knew Cooley had not heard from him in a while, and they called the building superintendent and asked him to check on the man.

The door was locked and when the superintendent came in with a key, Cooley was found face up on the couch dead with no clear signs of injury, sources said. The apartment had not been ransacked, and the only sign that anything was wrong was a lamp that was knocked over, source said.

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At the time, police suspected Cooley might have had a heart attack, grabbed the lamp for balance, and fallen on the couch, sources said. He was suspected to have been dead for a few days.

But on Friday, Chief Medical Examiner Dr. Barbara Sampson ruled that Cooley had been strangled in a homicide.

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The investigation continued late Thursday.