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TOKYO (AP) _ An explosion was reported Monday outside a U.S. Army base in Japan, the Pentagon said, and Japanese media reported that a projectile launcher was found in the area.

Police suspected it was an attack and that leftist radicals may have been involved, according to Japan's Kyodo news service. No injuries or damage were reported, Kyodo said.

Maj. Timothy Blair, a Pentagon spokesman, said he was told of an explosion about 800 feet from Camp Zama. The base, headquarters for the U.S. Army Japan and the 9th Theater Support Command, is located outside Tokyo.

According to Kyodo, several explosions were heard in a park near the base about 11 p.m. Police found the launcher and burn marks nearby, indicating that it had been fired, the news service said.

Police and military officials were not immediately available for comment.

Leftist radicals in Japan are known for using similar projectile launchers in attacks on targets related to the U.S. military here or on targets related to the royal family.

An explosion was reported in February near a baseball field at a high school in Kanazawa Ward of Yokohama, just south of Tokyo, near the U.S. Navy's Koshiba Fuel Terminal.

There are about 50,000 U.S troops stationed in Japan under a mutual security pact.