(AP) — A northern Illinois chemical plant that was rocked by an explosion and massive fires that prompted evacuations was inspected by a federal agency less than a month before the blast sent debris raining down onto nearby areas.

Inspectors from the U.S. Labor Department’s Occupational Health and Safety Administration on May 20 investigated a complaint at Chemtool Inc., a plant northwest of Chicago, WLS-TV reported. Fires continued to burn Tuesday following Monday’s explosion.

A summary record of that inspection does not explain what was being inspected at the Rockton-area plant that manufactures lubricants, grease products and other fluids. That record states only that the complaint involved safety and health, and the case remains open, the station reported.

OSHA officials said they have six months to complete their investigation involving the plant, and no further information will be made public until that work is finished, WLS-TV reported. There are no other outstanding cases or any violations in U.S. records pertaining to the plant.

A company spokesperson said that Chemtool has been safely operating since 2008, employing about 200 employees, and Chemtool said all employees on site “at the onset of the fire got out without anyone getting hurt.”

Chemtool has no U.S. Environmental Protection Agency violations during at least the last three years, the station reported.

Fires continued burning Tuesday at the ruins of the plant, where helicopter footage showed plumes of dark smoke still rising as fire crews directed water onto a portion of the smoldering site. Blackened and collapsed portions of the plant were visible along with fire-damaged semi-trailers.

A message seeking an update on fire crews’ efforts was left Tuesday for a spokeswoman with the Rockton Fire Department.

Fire Chief Kirk Wilson said Monday that about 70 employees were evacuated safely from the plant, and that one firefighter suffered a minor injury following the explosion. Chemtool’s parent company, Lubrizol Corp., later said there were closer to 50 employees present when the plant was evacuated.

Fire officials had ordered a mandatory evacuation of homes and businesses near the plant, but later in the day, Gov. J.B. Pritzker’s office expanded the evacuation zone from a 1-mile (1.6 kilometer) radius to a 2-mile (3.2-kilometer) radius from the plant “out of an abundance of caution.” Pritzker’s office also encouraged people within that zone to wear masks to prevent them from inhaling soot.

Rockton is located in Winnebago County, near the Wisconsin border, about 95 miles (150 kilometers) northwest of Chicago. Trisha Diduch, the planning and development administrator for Rockton, said Monday that she estimated that about 1,000 people were affected by the evacuation order.

A message seeking an update on the evacuations was left Tuesday for Rockton officials.

Chemtool, which touts itself as the largest manufacturer of grease in the Americas, has been in business since 1963. Its was purchased nearly eight years ago by the global chemical firm Lubrizol, owned by Berkshire Hathaway.