CHICAGO (AP) — Officials are investigating the events surrounding a power failure at a nuclear reactor in northern Illinois, where steam was vented to reduce pressure after it shut down.

After the shut down Monday morning at Exelon Nuclear’s Byron Generating Station, operators began releasing steam to cool the reactor from the part of the plant where turbines are producing electricity, not from within the nuclear reactor itself, officials said. The steam contains low levels of tritium, a radioactive form of hydrogen, but federal and plant officials insisted the levels were safe for workers and the public.

Diesel generators were supplying the reactor with electricity, though it hasn’t been generating power during the investigation into what happened. One question is why smoke was seen from an onsite station transformer, though no evidence of a fire was found when the plant’s fire brigade responded, U.S. Nuclear Regulatory Commission spokeswoman Viktoria Mitlyng said.

Exelon Nuclear officials believe a failed piece of equipment at a switchyard at the plant about 95 miles northwest of Chicago caused the shutdown, but they were still investigating an exact cause. The switchyard is similar to a large substation that delivers power to the plant from the electrical grid and from the plant to the electrical grid.

The commission declared the incident an “unusual event,” the lowest of four levels of emergency.

“It was standard procedure that they would notify county officials,” she said. “There is always concern. But, it never crossed my mind that there was any danger to the people of Ogle County.”

Another reactor at the plant was operating normally.