STOCKTON, Iowa (AP) — An autopsy shows an Illinois scuba diver who died inside a million-gallon farm tank in eastern Iowa accidentally drowned.

The Gazette of Cedar Rapids reports that the autopsy for 54-year-old Robert Baenziger Jr., of East Moline, Illinois, was released Wednesday.

It shows Baenziger died June 8 in an anaerobic digester, a large tank in which cow manure and food waste are combined with water and broken down by microorganisms to create methane.

Officials had initially described the tank as a manure tank. Baenziger was a self-employed contractor exempt from federal workplace oversight who had been hired by Sievers Family Farm in Stockton to make repairs inside the tank.