Japanese authorities were investigating Wednesday after a small drone was found on the roof of the prime minister's office.

No injuries or damage were reported from the incident, and Prime Minister Shinzo Abe was in Indonesia to attend an Asian-African conference.

Police said it was not immediately known who was responsible for the drone. They were investigating the possibility it had crashed during a flight.

It was not clear when the drone landed. It was found Wednesday by an official who was taking new employees on a tour of the prime minister's office in central Tokyo, according to media reports.

What was initially considered only a mishap turned eerie when reports said officials detected a trace of radiation from the drone, presumably not levels harmful to humans. TV video showed several unformed policemen without hazmat suits carrying a blue plastic box containing the drone for further examination.

Video from public television broadcaster NHK earlier showed dozens of police officers and officials around the drone, which was covered by a blue tarp.

The drone was about 50 centimeters (1.7 feet) in diameter and had four propellers, carrying a small camera and something that looked like a flare, NHK said. It was also decorated with a symbol that warns of radioactive material.

Small drones are becoming increasingly popular in Japan and are often used for performances, aerial filming and other purposes, but have been raising safety concerns.

In the U.S. in January, a wayward drone flown by an off-duty intelligence employee crashed on the White House grounds, raising questions over how commercial and consumer drones can be used safely in the U.S.

Japanese aviation laws have no restrictions for flying unmanned equipment at or below 250 meters (820 feet) above ground except for flight routes.