Investigators gather around a small drone covered with blue sheets, foreground, of the roof of Prime Minister Shizo Abe's official residence in Tokyo Wednesday, April 22, 2015. Japanese authorities are investigating the small drone that had landed on the roof. No injuries or damage have been reported from Wednesday's incident, and Abe is away, on a visit to Indonesia to attend the Asia-African Conference. Police said it was not immediately known who was responsible. They are investigating a possibility it had fallen during a flight. (Tsuyoshi Ueda/Kyodo News via AP) JAPAN OUT, MANDATORY CREDIT
Tsuyoshi Ueda/Kyodo News via AP
Investigators gather around a small drone covered with blue sheets, foreground, of the roof of Prime Minister Shizo Abe's official residence in Tokyo Wednesday, April 22, 2015. Japanese authorities are investigating the small drone that had landed on the roof. No injuries or damage have been reported from Wednesday's incident, and Abe is away, on a visit to Indonesia to attend the Asia-African Conference. Police said it was not immediately known who was responsible. They are investigating a possibility it had fallen during a flight. (Tsuyoshi Ueda/Kyodo News via AP) JAPAN OUT, MANDATORY CREDIT
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Story highlights

The drone is sparking terrorism concerns, authorities say

It was equipped with a bottle containing radioactive material

It was discovered as a court approved a plan to restart two Japanese nuclear reactors

(CNN) —  

A bizarre and alarming discovery is raising concerns in Japan about the potential for terrorism involving drones.

A drone carrying traces of a radioactive material was found on the rooftop of Japan’s equivalent to the White House on Wednesday, police and government officials said.

The discovery came on the same day a Japanese court approved a government plan to restart two reactors at the Sendai nuclear power plant in Kagoshima prefecture, more than four years after the Fukushima Daiichi nuclear disaster.

Prime Minister Shinzo Abe’s push to restart the reactors is unpopular among many Japanese, who view nuclear energy as too dangerous.

A staff member spotted the drone Wednesday morning on the roof of Abe’s residence, Tokyo Metropolitan Police said.

Prime Minister Shinzo Abe was in Indonesia at a conference when the drone was spotted.
Kyodo News via AP
Prime Minister Shinzo Abe was in Indonesia at a conference when the drone was spotted.

Dozens of police investigators were dispatched to the roof to investigate the origin of the drone, which had four propeller and was 50 centimeters (20 inches) wide.

Police say the drone was equipped with a small camera, smoke flares and a plastic bottle containing small traces of a radioactive material believed to be cesium, a common byproduct of nuclear reactors.

Cesium was also discovered in areas around the failed Fukushima Daiichi nuclear plant after its 2011 meltdown.

Investigators suspect the cesium was placed in the bottle. The amount inside is not immediately harmful to humans.

Chief Cabinet Secretary Yoshihide Suga said the discovery is raising concerns about terrorism.

“There might be terrorism attempts in the future at the Olympics and G7 Summit using drones,” Suga said. “So we need to examine and review continuously the way small unmanned vehicles like drones should be operated and how to cope with the threat of terrorism from drones. The government will do all that we can to prevent terrorism.”

Japanese law restricts drone flights around airports to prevent problems with aircraft, but there are no flight restrictions for most of Tokyo, including the Prime Minister’s residence and local and federal government buildings.

Abe was not in his office at the time. He is in Indonesia, attending the Asian-African Conference.

CNN’s Elizabeth Joseph, Joshua Berlinger and Josh Levs contributed to this report.