Police raided locations linked to an ultra-leftist group in 10 prefectures Wednesday in connection with attacks against the Defense Agency and U.S. military bases, government sources said.

Investigators strongly suspect that a dissident faction within the Kakurokyo group was involved in an attack using a homemade mortar against the Defense Agency's headquarters late Tuesday night, the sources said, without providing information on the outcome of the searches.

The launchers used in Tuesday's attack closely resemble weapons employed in earlier attacks allegedly carried out by the extremist group, the sources said.

The group released statements claiming responsibility for mortar attacks against the agency and the U.S. military's Yokota Air Base, in suburban Tokyo, last March at the height of a national debate over the U.S. plan to attack Iraq.

The group also claimed responsibility for an attack April 3 at the U.S. Naval Air Facility at Atsugi, Kanagawa Prefecture.

Around 11 p.m. Tuesday, loud explosions were heard twice in Shinjuku Ward near Zuikoji Temple, which lies 550 meters from the Defense Agency's headquarters.

In the cemetery of the temple, police found two tubes apparently used to launch projectiles. The launchers were reportedly equipped with a timing device.

The Defense Agency mobilized 100 members of its police unit to search the agency compound but had not found any projectiles or any other suspicious objects as of Wednesday afternoon, the Defense Agency said.

Police have been on high alert recently, with stepped up security at the Diet and Prime Minister's Official Residence.