TOKYO -- Japanese police said Friday they are investigating a man claiming responsibility for landing a drone on the roof of the Japanese Prime Minister's Office this week.

Tokyo's metropolitan police said the man turned himself in to the Fukui prefectural police in western Japan.

The small drone found Wednesday had traces of radiation and triggered fear of potential terrorist attacks using unmanned aerial devices.

Police have not determined if the man was responsible for the drone or had broken any law. He hasn't been arrested, and his identity has not been released.

Kyodo News agency said a police official quoted the man as saying he flew the drone to protest the government's nuclear energy policy.

Nobody was injured and Prime Minister Shinzo Abe was travelling during the incident.

The drone was carrying a small camera and a plastic bottle containing what police suspected was the source of radioactive cesium, levels of which were too low to affect humans or the environment.

It was not clear exactly when the drone landed because workers at the office in central Tokyo rarely go up to the roof. An official reportedly taking new employees on a building tour happened to spot the drone.