IDAHO FALLS, Idaho (AP) — A new report says a 2014 radiation leak at an Idaho research facility was caused by compromised equipment and faulty air monitors.

The Post Register reports that the 43-page internal report says radioactive material escaped from a sealed compartment at Idaho National Laboratory and remained undetected for several days. The leak at INL's Materials and Fuel Complex west of Idaho Falls wasn't discovered by employees until routine testing of air filters about a month later. Nine workers were found to have been internally exposed to the material.

The report says their dose levels were well below the yearly regulatory limits. Former INL Director John Grossenbacher said last year that the leak was caused by materials behaving in unexpected ways. The room and equipment where the leak occurred returned to normal operations in January.