The discovery came during a wellness check, when police forced their way into the man's second-floor apartment on 101st Avenue just before 1 p.m. Wednesday, triggering a large response by multiple city and federal agencies.
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The man was found sitting on a chair, and he is believed to have been dead for at least a week.
As police searched his apartment, they discovered canisters with HazMat and biohazard labels on more than one.
Some canisters were specifically labeled, but police are uncertain if they actually contained those chemicals.
They also found schematics of unknown infrastructure, leading police to question why the man was in possession of the materials.
Several agencies were called into the investigation, including agents with the FBI-NYPD's Joint Terrorism Task Force.
The city Department of Environmental Protection conducted air-quality samples and did not detect any hazardous materials in the air, so an evacuation of nearby residents was not deemed necessary.
City workers were set to return to remove the canisters and conduct tests on their contents.
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Officials donned hazardous material suits to remove the man's body.
The medical examiner will determine his cause of death.
Authorities said the man was a military veteran who was not on any terror watch list and did not have any known terror links.
He has one prior arrest from a long time ago, and he lived alone, separated from his wife decades ago.
Law enforcement is leaning towards him being a harmless eccentric, but the investigation continues.
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They are searching through the documents, with drawings and statements about chemicals, as well as testing the substances in the containers.
Officials said that on one of the pieces of paper, he had written "Warning - Danger. Don't open this. Why die?" They believe this to be referencing a neighborhood understanding that he would attack with chemicals anyone who attempted to break into his apartment.
He has not been identified, pending family notification.
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