Suffolk police detonated an apparently harmless section of PVC pipe found at Strathmore Park in Stony Brook on Sunday.
Police conducted the controlled detonation, known as a "render safe" operation, after a jogger reported seeing a suspicious device in the park.
Nearby Sycamore Drive, which briefly closed because of police activity Sunday afternoon, was reopened by 1:30, a police spokeswoman said.
Suffolk's Arson Section is investigating the incident. Emergency Services officers responded and detonated the explosion.