The operator of Japan’s crippled Fukushima nuclear plant said on Monday two workers were found to be contaminated with radioactive particles, the second such incident in a week involving staff outside the site’s main operations centre.

Two dust monitors sounded alarms earlier on Monday outside the operations centre, where radiation levels are usually low enough to avoid the need for full face masks, Tokyo Electric Power Co said in an emailed statement.

Two workers waiting for a bus at the end of their shift were found to be have been contaminated with radioactive particles, which were wiped off their bodies before they left the site, Tokyo Electric, also known as Tepco, said. Full body checks of the staff members showed no internal contamination.

The utility said it could not be sure the alarms were connected with the discovery of the contamination of the workers. The incident is being investiged.

Last week, the same monitors sounded alarms and 10 workers waiting for a bus were found to have been contaminated with particles. Tepco said it suspected they came from a mist sprayer used to cool staff down during the current hot summer.

The mist sprayer has been turned off since last week.

The Fukushima plant was crippled by an earthquake and tsunami in March 2011, the world’s worst nuclear disaster in a quarter century.

The government said this month it would step up its involvement in the plant’s cleanup after announcing the station was leaking contaminated water into the ocean, following months of denials of a leakage by Tepco.