Two improvised rocket launchers have been fired near a US military base near Tokyo, police say; no one was hurt and no damage was done.
Suspicions have fallen on a small extreme left group, media reported, with the incident coming hours after a senior member was arrested.
"The incident occurred at 11.30pm on Thursday. Flying objects were launched near the US military's Yokota airbase," a spokesman for Tokyo Metropolitan Police said.
"Several reports were made by neighbours that they had heard explosions at about 11.30pm. Police discovered two steel pipes, two wires, batteries, and so forth.
"No damage has been reported so far and police have not found the projectiles."
Local media reported on Friday the pipes were buried in the ground, with the upper end pointing in the direction of Yokota Air Base.
A statement from the US military confirmed the presence of "an improvised mortar launch outside Yokota".
"There were no injuries and thus far we have found no damage or impact points here on base. We take matters of security at our installation very seriously and we are assisting" police, it said.
There have been no claims of responsibility, but police believe it may have been a guerilla attack by extreme leftists, Jiji Press and other media said.
Broadcaster NTV reported police were concentrating their investigations on a hard leftist group because the incident came just hours after they arrested a top-ranking member on Thursday.
It named the group as the Revolutionary Labourers' Association Anti-Majority Faction.
A similar incident occurred four years ago, when police raided hideouts of the same leftists, NTV said.
Japan's politics is generally moderate, although there are vestiges of the extreme radicalism on both left and right that abounded in the 1960s and `70s.
One of the most famous groups was the United Red Army, which gained notoriety for its extreme brutality despite its small size.
A linked group, the Japanese Red Army, operated from Lebanon in tandem with Palestinian militias, carrying out notorious terror attacks including a massacre at Israel's Lod airport and hijackings of Japan Airlines planes.
Thursday's incident comes as tensions run high in the region between China and Japan with the focus on a group of disputed islands in the East China Sea.
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