GRAND RAPIDS, Mich. (WOOD) — A quarter of all potentially dangerous storage tanks in Michigan are overdue for inspection, according to a state audit released Wednesday.

There are more than 23,000 above-ground and underground storage tanks in Michigan that contain flammable compressed gas, liquified petroleum gas, flammable or combustible liquid or other petroleum-based or hazardous substances. They’re supposed to be inspected every three years.

Auditors based their findings on a small sample of inspection records. They also found that re-inspections were late on half the tanks that already had safety violations.

In 78% of the cases, inspectors had failed to put red tags on tanks with violations. The red tags show that those tanks should not be filled.

The Bureau of Fire Services within the Michigan Department of Licensing and Regulatory Affairs agreed with the findings and told auditors it is trying to find ways to be more efficient.