TOKYO — Kanagawa Prefectural Police on Wednesday raided operational bases of a Japanese extreme leftist group believed to have been involved with shooting projectiles at Yokosuka Naval Base in September, according to a police spokesman.

Police raided 12 Kakurokyo bases in eight prefectures and confiscated about 60 items, including group publications and handouts, the spokesman said.

No arrests have been made, he said.

Explosions were heard around 10:30 p.m. on Sept. 12 in a hilly, wooded area southwest of the U.S. Navy base near Shioiri Station.

Police found two 39-inch-long metal pipes and burned lead wire. One pipe was buried in the ground with 8 inches of it showing above the surface. No projectiles were found. No injuries were reported, but a 4-inch-diameter hole was found on the balcony of a nearby home.

Kakurokyo later claimed responsibility in a letter sent to various newspapers stating the attack was the first of an effort to stop the Navy from forward deploying the aircraft carrier USS George Washington to Japan. The nuclear-powered carrier arrived at Yokosuka Naval Base on Sept. 25.

The group also claimed responsibility for other explosions that targeted the military, including a 2007 blast near Camp Zama signaling the group’s opposition to a plan to move the Army’s I Corps (Forward) headquarters to the Tokyo-area base.

Kakurokyo is one of about 50 "New Left" groups operating in Japan, according to the U.S. Department of Homeland Security’s National Consortium for the Study of Terrorism and Responses to Terrorism, based at the University of Maryland.

The majority of such groups were formed after the 1955 decision of the Japanese Communist party to abandon violent tactics. Some members, however, chose to start new groups that remained committed to "terrorist" actions, according to the consortium’s Web site.