This is coolbert:
The culprit has turned himself in. As with the drone found on the White House lawn the miscreant is a disaffected person with grievance, a drunk or just a plain old ordinary dumb idiot.
"‘Protest against nuclear policy’: Japan’s radioactive drone pilot turns himself in"
"A 40-year-old Japanese man has taken responsibility for launching a drone with radioactive material that landed on the roof of the office of Prime Minister Shinzo Abe. He said he took the course of action to protest the government’s nuclear policy."
. . . .
"A Chinese-made Phantom drone was found near the helicopter-landing pad above Abe’s office on Wednesday. However, it could have been there for several days before it was eventually discovered. The 50 cm diameter remote controlled device was clutching a water bottle containing liquid with trace amounts of cesium - a radioactive compound."
Trace amounts only! OH, I am glad it is trace amounts only!