ST. PAUL — The Minnesota Department of Health said Tuesday it is requiring nearly 600 nursing assistants to retake their registration exams after an investigation found “potential improprieties” at two Inver Hills Community College sites.

The unusual move follows an investigation into a “suspicious pattern in test results” at Inver Hills’ Center for Professional and Workforce Development, and its satellite location at Blue Sky Online in West St. Paul, the Star Tribune reported. Minnesota State Colleges and Universities joined in the announcement.

The requirement affects all students who took tests at the two sites between May 1, 2014, and Oct. 16, 2015, to become federally qualified nursing assistants — regardless of whether their specific test results were questionable.

State health officials are now contacting nursing assistants who need to retake the test, at no cost to them, along with their employers.