"Unknown contaminant" (?) causes nitric acid tank release of NOX in Ohio

Via Ohio TV station WXIX, this chemical news: 

VINTON COUNTY, Ohio (WXIX) - Residents in Vinton County are back in their homes after they were evacuated due to a chemical release at a southeast Ohio explosives manufacturing facility.

Emergency crews issued an evacuation order within a three-mile radius of the Austin Powder Red Diamond Company in McArthur Wednesday morning after a 5,000-gallon tank with nitric acid released 3,000 gallons of nitric oxide into the atmosphere.

According to the Ohio Environmental Protection Agency, an “unknown contaminant entered the nitric acid tank,” causing the chemical reaction to occur.

Although the release was stopped, the lingering chemical plume prompted an evacuation of the small Village of Zaleski and several roads surrounding the Austin Powder Plant. Residents were able to go home at 4:40 p.m.

I too do not know what the contaminant was, but I'm guessing it was water or air. No fun for the local birds for sure.