A drone carrying small traces of a radioactive material was found Wednesday morning on the roof of Japanese Prime Minister Shinzo Abe's office, police said, according to CNN.

A member of the Prime Minister's staff spotted the four-propeller, 50-centimeter wide drone and alerted authorities, Tokyo Metropolitan Police said.

The drone was equipped with a small camera and a plastic bottle containing small traces of a radioactive material, police said.

The material was cesium, a soft metal, and what was found was harmless to the human body, police said. Two smoke flares were attached to the drone.

Abe was not in his office at the time. He is in Indonesia, attending the Asian-African Conference.