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It admitted launching two projectiles at Camp Zama at around 11 p.m. Monday and said the launch was "an attack to prevent" realignment of the U.S. military in Japan and U.S. Vice President Dick Cheney's three-day visit to Japan starting Tuesday.

The realignment plan includes transformation of the U.S. Army's command and control structure at Camp Zama.

The statement said the attack follows a similar incident in 2002 when two rockets were launched at the U.S. base, adding that this time "a new type of mortar projectile" was used.

A similar statement was sent by the group in 2002 to claim responsibility for the earlier incident.

During the investigation of the latest case, in which no one was injured, the police have found fragments of what they believe to have been a timing device and two metal tubes believed to be launchers near the camp, some 40 km southwest of Tokyo.

U.S. television news outlet ABC News said on its Web site earlier in the week that the explosions could have been the first attempt by an al-Qaida terrorist cell to launch an attack in Japan.