The U.S. Environmental Protection Agency has launched a criminal investigation into a large leak of ammonia from a Columbia County fertilizer plant in 2010 that went undetected for five days.

Records show the plant—owned by an Australian chemical company—has a history of EPA penalties, including for other ammonia leaks.

EPA officials won't confirm the criminal investigation. But managers at the Dyno Nobel plant in Deer Island, about 35 miles north of Portland on U.S. Route 30, told employees about the investigation in a memo obtained by WW. The memo says employees are not required to answer investigators' questions, and they should refer all inquiries to the company's attorneys.

Plant manager Greg Godfrey confirmed to WW that EPA criminal investigators interviewed workers Jan. 23 about the leak.

"Because it's a criminal case, I'm assuming that they think we intentionally violated the law, which isn't the case," Godfrey says.

State and federal environmental officials occasionally impose civil fines on companies, but criminal cases are not common.

The plant's parent company, Incitec Pivot Ltd., is an Australian chemical company that posted $3.9 billion in sales last year. Its subsidiary, Dyno Nobel, is one of the world's largest makers of explosives; its predecessor was founded by Alfred Nobel, the inventor of dynamite and the international prizes that carry his name.

The Oregon plant with 65 employees produces agricultural-grade nitrogen fertilizers, as well as chemicals used to reduce pollution from power plants.

Officials for the state Department of Environmental Quality say ammonia isn't a regulated pollutant, so neither the state nor federal regulators have limits on its release into the air.

But ammonia is covered by a federal law that requires companies to notify the EPA right away if unexpected releases of dangerous chemicals occur.

DEQ records reviewed by WW include a brief account of what Dyno Nobel said happened at its Deer Island plant in 2010: The plant shut down after a power outage on Aug. 29. A valve was left open when the plant restarted two days later, leaking ammonia. 

Records show neighbors complained about an ammonia smell. Dyno Nobel told the EPA that its technicians couldn't detect a leak. Five days after the plant restarted, however, a company inspector doing a routine check found the open valve. 

By that time, an estimated 24.6 tons of ammonia had escaped. That's more than 100 times bigger than a leak at the plant in 2008 that brought a $17,000 civil fine from the EPA. 

The EPA alleged that Dyno Nobel took 11 hours to report the 2008 leak. The EPA settlement required Dyno Nobel to install new leak-detection equipment.

Godfrey says the plant did so, but the detectors did not pick up the 2010 leak. He said plant operators reported the leak as soon as it was discovered.

"Why they're looking at us with a criminal investigation, that's a mystery to us," he says. “We did everything that was required by the law.”