Norway's role in Afghanistan and its decision to file terrorism charges against a Muslim cleric may explain the Oslo bomb attack

Oslo police have confirmed the source of the blast that damaged the prime minister's offices in Oslo was a bomb. The question now is who is likely to be behind it.

The most obvious conclusion would be a jihadist group.

It has been known for some time that al-Qaida and other related "franchises" – including the most active groups in Yemen – have been trying to develop operations. Which leads to a second question: why Norway?

The answer is threefold: In the first instance, with increased levels of security and surveillance in the UK and the US as well as other European capitals, Norway might have been seen as a softer target despite the recent breaking up of an al-Qaida cell in Norway.

A more detailed explanation of Norway's problems with al-Qaida were supplied a year ago by the Atlantic magazine in an article by Thomas Hegghammer, a senior fellow at the Norwegian Defence Research Establishment in Oslo, and Dominic Tierney .

The article followed the arrest of three men in Norway and Germany for allegedly plotting a terrorist attack involving peroxide explosives. All were Muslim immigrants to Norway.

"The first explanation," the article stated, "is Afghanistan. Norway has been part of the International Security Assistance Force (Isaf) in Afghanistan from its foundation in late 2001 ... In late 2007, for example, Ayman al-Zawahiri, al-Qaida's second-in-command, said the group had previously threatened Norway because it participated in the war against the Muslims."

However, the article added, "Norway is, at best, a secondary player in Afghanistan. Most of its soldiers are in the northern cities of Meymaneh and Mazar-i-Sharif, a relatively quiet part of the country. And its contingent of 500 personnel is only the 18th largest in the Isaf coalition."

A second possible factor behind the attack is a Norwegian newspaper's reprinting in 2006 of a series of Danish cartoons depicting the Prophet Muhammad, which prompted threats against the country.

A third potential explanation is the decision last week by a Norwegian prosecutor to file terror charges against an Iraqi-born cleric for threatening to kill Norwegian politicians if he is deported.

The indictment centered on statements that Mullah Krekar, the founder of the Kurdish Islamist group Ansar al-Islam, made to various media, including the American network NBC.

A report earlier this year by the Norwegian intelligence noted the increasing risk of a jihadi terrorist attack, with Norwegian citizens reportedly trained in Pakistan, Somalia, Yemen and Afghanistan.

"Although few people in Norway support Islamic extremism, there are activities within some groups that could contribute to heightening the security risk in 2011," the report said.

Some individuals in Norway have travelled to take part in training or fighting" in countries such as Afghanistan, Pakistan, Somali and Yemen, the report added.