Northampton County Coroner Zachary Lysek announced the findings with investigators, who said numerous syringes and needles were discovered in a gym bag in Garrett Reid's room.
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Garrett Reid was found dead in his dorm room on Aug. 5 at Lehigh University in Easton, where he was assisting the team's strength and conditioning coach during training camp.
He was a recovering drug addict who had seemingly turned his life around. His father, Andy Reid, had indicated that his 29-year-old son may have had a relapse.
Northampton County District Attorney John Morganelli said that investigators found 47 syringes and 65 needles in the younger Reid's gym bag, many of them unopened.
Also found were syringes and spoons, too.
"We are confident today that Mr. Reid's death was the result of a self-injected lethal dose of heroin," he said.
Morganelli said the investigation is now examining from whom Garrett Reid got the narcotics and the drug paraphernalia.
He also said that besides the heroin, investigators found 19 vials of an unknown liquid, but did not say what they contained.
"We're not sure, exactly, what those substances are at this time," he said, adding they were also in the bag but had nothing to do with Garrett Reid's death.