A massive chemical plant fire that sent black plumes of smoke into the air near the Illinois-Wisconsin border is expected to continue burning for several days as firefighters retreat and residents were ordered to evacuate.

Fire brokeout around 7 a.m. at the Chemtool Inc. plant, which makes oil-based lubricants, at 1165 Prairie Hill Road in Rockton, about 10 miles north or Rockford, Rockton Fire Protection District Chief Kirk Wilson said.

The cause of the fire remains unknown, and no significant injuries were reported.

“We do not yet know what caused this incident, but we will be working with local authorities and with our own risk management team to determine what happened and identify any corrective actions,” Chemtool said in a statement.

Firefighters stopped spraying the fire with water to reduce the risk of runoff and contamination of waterways.

“We don’t want an environmental nightmare to occur,” Wilson said in a news conference Monday. “At this time, it’s in our best interest to let this product burn off.... We’re thinking this will be a several-day event.”

As a precaution, residents were evacuated within a mile of the plant, he said. So far, air quality tests show the air is OK at ground level, Wilson said.

An employee of the plant told a reporter with WTVO that he was at work for less than an hour when someone yelled “fire” and alarms went off. The employee said the fire was possibly caused by the failure of a pipe that moved hot grease within the plant, WTVO reported.

The employee also told WTVO that staff have been evacuated in the past for small emergencies, but nothing on this scale.

“This is the worst it’s ever, ever been,” he told the station.

Gov. J.B. Pritzker deployed the Illinois National Guard and activated the state’s emergency center to the fire.

“I am monitoring this situation closely and will make all resources available to the surrounding communities as we work to keep people safe,” Pritzker said in a statement.

The U.S. Environmental Protection Agency said it responded to the site to conduct air monitoring and sampling. Officials recommended people within a three-mile radius wear masks as a precaution.

Smoke from the fire drifted south and southeast past Rockford, which warned residents to stay indoors, close windows and turn off air conditioners. South of Rockford, the Ogle County sheriff’s office said smoke covered a third of the county.

About 70 employees evacuated the plant by themselves and were unharmed, he said. One firefighter suffered a minor, unspecified injury. About 175 personnel from over 40 local fire agencies were called to assist, he said.

The fire department also has reconnaissance crews roaming the town looking for fires caused by falling burning debris, Wilson said. Any debris falling from the sky was thought to be non-toxic, he said.

In all, about 150 homes were evacuated in Rockton. The Salvation Army said it was aiding in the response.

Chemtool Inc. is the largest manufacturer of grease in the United States. It has a plant in far northwest suburban Crystal Lake, too.

Contributing: Brett Chase, AP