Japanese authorities launched an investigation after discovering a small drone laced with traces of radiation Wednesday on the roof of Prime Minister Shinzo Abe's office.
Abe was away from his office at the time, attending a conference in Indonesia. No injuries or damage was reported.
The drone was about 50 centimeters (1.7 feet) in diameter, had four propellers, and carried a small camera and a plastic bottle with unidentified contents, Tokyo police said. The bottle, they said, was believed to be the source of radioactive cesium contamination.
The drone was also decorated with a symbol that warns of radioactive material, according to national broadcaster NHK.
Police said the level of radiation detected was not harmful to humans. Television footage showed dozens of police officers hovering around the drone, which was covered by a blue tarp. TV video later showed several uniformed policemen removing the drone for further examination. They did not wear protective suits as they carried the drone away in a blue plastic box.
It was not clear who sent the drone or what the motive may have been. It also was not clear when the drone landed. Officials at the prime minister's office, located in central Tokyo, said they rarely go up to the roof, which is also used as a heliport in case of an emergency. News reports said the drone was found by an official who was taking new employees on a tour of the prime minister's office.
Chief Cabinet Secretary Yoshihide Suga said the drone incident was a wake-up call regarding problems that unmanned aerial devices could cause at such events as next year's Group of Seven summit in Japan or the 2020 Tokyo Olympics.
"There is a possibility that drones might be used for terrorist attacks,'' Suga said at a news conference. "Taking into consideration the latest incident, we will review drone use and measures for possible terrorist attacks using drones. We'll do our utmost to prevent terrorist attacks.''
Some information for this report came from AP.