TOKYO -- Japanese police have arrested a man who admitted to landing a drone with low-level radioactive sand on the roof of the prime minister's office to protest the government's nuclear energy policy, officials said Saturday.

Tokyo metropolitan police said Yasuo Yamamoto, 40, turned himself in to authorities late Friday in Fukui in western Japan.

The small drone found Wednesday had traces of radiation and triggered fears of potential terrorist attacks using unmanned aerial devices, the prime ministers' office has said. The infiltration at Japan's political headquarters has also raised questions over the level of security there.

No one was injured and Prime Minister Shinzo Abe was travelling at the time of the incident.

Police said Yamamoto was arrested on suspicion of flying the drone and obstructing duties at the prime minister's office. He has not been indicted and faces further questioning.

Public broadcaster NHK said police quoted the unemployed man as saying he did it to protest the government's nuclear energy policy.

Fukui is home to about a quarter of Japan's 48 reactors, which are currently all offline following the 2011 tsunami-triggered Fukushima plant disaster. Abe's administration wants to restart as many of the idled plants as possible.

The drone was carrying a small camera and a plastic bottle containing what police suspect was the source of radioactive cesium, levels of which were too low to affect humans or the environment.

The suspect said he used the sand from an unspecified location in Fukushima, where radiation levels are still high due to fallouts from the Fukushima Dai-ichi meltdowns, NHK said.

The government has set up a task force to work on regulations for the use of small drones while ensuring the security of key government facilities. Drones have become increasingly popular in Japan for industrial, service and entertainment use. At a meeting Friday, task force members cautioned against excessive restrictions on the emerging market for drones.

It is not clear exactly when the drone landed because workers at the office in central Tokyo rarely go up to the roof. An official taking new employees on a building tour reportedly spotted the drone.

A blog with an anti-nuclear theme managed under the name "Kantei (prime minister's office) Santa" contained photos of a drone that looked like the one found Wednesday. In one entry, the anonymous writer said he drove from his hometown in Obama city in Fukui to Tokyo and flew the drone in the early hours of April 9, trying to have it land in the yard of the prime minister's office, but that he eventually lost track of the device.

In the United States in January, a drone flown by an off-duty intelligence employee crashed on the White House grounds.