A US military installation in Japan was threatened with a small explosion outside the camp:

A small explosion occurred outside a U.S. Army base south of Tokyo late Monday, police and military officials said. A Japanese news report said police suspected an attempted attack on the base.

The Army was investigating the blast, said an official at Camp Zama who spoke on condition of anonymity, adding that there were no reports of injuries or damage.

"A small explosion was heard in the vicinity of the base," said Maj. David Smith, a Pentagon spokesman in Washington D.C. "It did not occur on the base."

Kyodo News agency reported that police had found a metal tube thought to be used as a launcher of a rocket from a nearby park. Investigators suspected guerrillas in the attack, the report said.

This isn’t the first time leftists have targeted a US military installation in Japan:

In 2002, two blasts were heard outside Camp Zama, and Japanese police found a metal projectile and a crude mortar made from a metal pipe nearby. Investigators blamed radical guerrillas for the explosions, which caused no injuries.

Leftist extremists in Japan have used projectile launchers against targets related to the U.S. military or on sites connected to the royal family. The attacks are usually more symbolic than dangerous, and injuries or significant damage are rare.

It appears it is probably the same leftist group judging by the device used.  As much as I hate South Korea’s fifth column, at least they have resorted to explosive yet, they appear content attacking people with sharpened bamboo spears and rocks.