The Loveland Reporter-Herald reports Coroner Carl A. Blesch released the determination Friday.
Kris Kazian, chief of Windsor Severence Fire Rescue, said responders on Wednesday extinguished a fire atop the tank. There, they found the man’s body.
A firearm was recovered from the scene.
Kazian has said previously that the man is believed to have been at the site alone performing maintenance.
The cause of the fire remains under investigation.