Five shipping containers detained in Rotterdam after testing positive for radiation probably left Japan before a screening program started, the Asian nation’s transport ministry said.

Japanese authorities officially began scanning cargo boxes for radiation on April 28 because of concerns about leaks from a nuclear plant crippled by a March 11 earthquake and tsunami.

“Japanese cargo owners have been screening containers before shipping them, so contamination should be rare,” said Hatsuho Tanaka, general affairs chief at the Japan Shipowners’ Association. “It’s possible that containers could get some contamination from the air or rain at sea.”

From Bloomberg