Attack bears hallmark of Japan's left-wing extremists
Julian Ryall in Tokyo
The foreign media buzz surrounding the apparent attack on Camp Zama might have been about al-Qaeda, but, to domestic defence analysts the somewhat amateurish attack bore the hallmarks instead of Japan's shrinking band of left-wing extremists.
'I have seen media reports that this may have been the work of al-Qaeda, but mortar attacks have been tried against US military bases in the past and have turned out to have been carried out by radical left-wingers,' said independent analyst Kensuke Ebata.
'These are people who used to be students but have grown older,' he said. 'They feel they need to carry on their struggle ideologically and have to show to the Japanese people that they are still fighting for their ideals.'
The attack may also have been an initiation rite for someone who wanted to join a leftist group, he suggested.
Japan's homegrown terrorist groups emerged from the revolutionary communist groups that were most visible and effective in the 1960s and early 1970s. The most influential group was the Japanese Red Army, which was set up in 1969 and carried out the Lod Airport massacre in Israel, the seizure of the French embassy in the Hague and attacks on the US and Swedish embassies in Kuala Lumpur. At home, its members were most famous for hijacking a Japan Airlines jet in 1970 and flying it to Pyongyang.
While members of the Red Army largely ended up in exile - until recent years, when more have been arrested at home and abroad - the Chukaku-ha focused its efforts on overthrowing Japan's monarchy and evicting the US military.
With around 3,500 members operating in cells, Chukaku-ha - which is often referred to as the Middle Core Faction - has managed to survive investigations by the Japanese police after previous mortar incidents and arson attacks, notably against officials connected to the construction of Tokyo's Narita Airport. The left joined forces with local farmers when the government carried out compulsory purchases of their land.
'Yes, American facilities have been the target of similar mortar attacks in the past, including one several years ago involving projectiles being fired at the embassy from the back of a truck,' said an official at the US embassy in Tokyo. He refused to comment on the likely culprits in this latest incident.
Describing the tactics used against Camp Zama as 'a primitive, small-scale attack', Mr Ebata said the assailants' main aim was probably to attract attention.
'Carrying out a major terrorist attack means that it is by definition difficult and dangerous, which means the people behind it will want to make sure it works,' he said.
'But it would be very difficult for al-Qaeda to get into Japan and get the explosives without being detected,' he said, adding that the blame this time must be pinned on ageing Japanese revolutionaries.