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.

WLS-TV reports that on May 20, inspectors from the U.S. Labor Department’s Occupational Health and Safety Administration investigated a complaint at Chemtool Inc.

A summary record of that inspection does not explain what was being inspected at the plant. A company spokesperson said that Chemtool has been safely operating since 2008. Fires continued burning Tuesday at the plant near the Wisconsin border, about 95 miles northwest of Chicago.