TOKYO — A small propeller-driven drone possibly tainted with radioactive cesium was discovered Wednesday on the roof of the prime minister’s office, unsettling officials, who said they were reviewing antiterrorism measures in response.

The officials said they did not know who had flown the drone, which was about two feet in width with four propellers. They also said they did not know how long the drone had been on the roof. No injuries or damages were reported. The Japanese prime minister, Shinzo Abe, was in Indonesia to attend an Asian-African summit meeting when the drone was discovered.

The police in Tokyo said that the drone was carrying a camera and a bottle of unidentified liquid that bore a sticker with the universal symbol for radioactivity.

The police also said traces of radioactive cesium were detected on the roof near the drone, though it was unclear if they came from the drone. Radioactive cesium is not usually found in nature, but large amounts of it were released into the atmosphere during the Fukushima nuclear disaster in 2011.