http://www.reuters.com/article/idUSTRE6A34DA20101104 Greek police believe radical left-wing guerrilla groups are responsible for a wave of parcel bombs sent to foreign governments and embassies in Athens this week. Following are some questions and answers about the bombing campaign: WHO ARE THE BOMBERS? Shortly after the first parcel bomb exploded on Monday at a courier company in Athens, police arrested two Greeks in their 20s, one of them a suspected member of the Fire Conspiracy Cells. No one has yet claimed responsibility for the attacks but police believe this to be the main group behind the bombings, though they say others may be involved. Greece has a decades-long history of leftist violence, and some groups have sprung up or become more active since December 2008, when the police killing of a teenager triggered three weeks of rioting and attracted global attention. Senior security officials believe the groups are all inter-related. The Fire Conspiracy Cells, which describe themselves as anti-state, initially specialized in arson attacks but turned to bombings in May 2009. Their devices usually contain small amounts of explosives, not enough to kill. Another new group, the Rebel Sect, claimed the killing of a Greek anti-terrorist policeman in June 2009 and of a journalist last July. Revolutionary Struggle, which says it aims to overthrow the existing economic and political order and is opposed to the influence of globalization and capitalism, has staged a number of bomb and grenade attacks since 2003. More than 20 suspected urban guerrillas have been arrested this year, but analysts say the groups' loose structure makes them hard to eradicate. Greece's deadliest guerrilla group, November 17, killed more than 20 people, including foreign diplomats, in 27 years before finally being dismantled in 2002. WHAT IS THE AIM? Sending parcel bombs, with no warning, is a new tactic, and carries the risk that uninvolved parties could be hurt. The choice of foreign targets appears to be aimed at attracting international attention without actually killing anyone -- police say the bombs can burn the person who opens the package, but are not big enough to kill. "They did not intend to cause casualties," said George Kassimeris, senior research fellow in conflict and terrorism at the University of Wolverhampton. "They want to dictate events by creating problems for the government and the security system, and send a message to international actors." The militants are almost certainly trying to tap into anger with an economic slump and the government's austerity policies -- imposed as the price of a 110 billion euro ($155 billion) bailout from the European Union and International Monetary Fund. They appear to have timed their attacks to try to boost an expected anti-government swing in this Sunday's local elections, as a first step toward creating political instability. But they may also be frustrated that anger with the government and its foreign creditors has not tipped over into the kind of rioting last seen two years ago. In December 2008, the police killing of a teenager triggered three weeks of street fighting and virtual economic standstill, contributing to the center-right government being voted out within a year. WHAT HAVE THEY ACHIEVED? While there have been numerous protest marches and demonstrations this year, the incidental death of three bank employees during a protest in May has so shocked ordinary Greeks that violence at protests is now barely tolerated. That shows no sign of changing for the time being. By sending packages as far as the office of German Chancellor Angela Merkel -- main paymaster of the Greek bailout -- the bombers have succeeded in grabbing headlines. But if they hoped to hurt Greece's creditworthiness and undermine the austerity programme, they appear to have failed, so far at least. Greek bond and stock markets did not react to the bombings, suggesting that investors have factored in the risk of sporadic guerrilla violence. Nevertheless, the guerrillas have provided a reminder that they are still a force to be reckoned with. The problems that are fuelling discontent -- economic crisis, youth unemployment and corruption -- are not likely to vanish soon, and analysts say more attacks could come.