Armed Police patrol outside the terminal building at Oslo Airport, Thursday, July 24, 2014.  (AP Photo/NTB Scanpix, Audun Braastad)

Norway's security police announced on Thursday that the country was facing an imminent terror threat. “The PST [Norway's Police Security Service] recently received information that individuals affiliated with an extreme Islamist group in Syria may have the intention of carrying out a terrorist action in Norway,” a statement posted to the PST's Web site reads, adding that the attack would most likely be "carried out shortly – probably in a few days."

For Norway, terrorist threats are not necessarily uncommon – the PST says that Norway learns of a great number of threats every year, though most are less credible. The Norwegian Special Forces fought in the war in Afghanistan, and Ayman al-Zawahiri, the head of al-Qaeda since the death of Osama bin Laden, has warned of an attack on Norway. Norwegian citizens and residents have been accused of taking part or planning terror events in Norway or at home – one of the suspects in last year's attack on a mall in Nairobi, Kenya, was a Norwegian citizen, and three men living in Norway were arrested for plotting terror attacks on Norway and Denmark in 2010. After a bomb struck Oslo and scores of people were shot dead on the island of Utoya, many rushed to assume Islamist terrorists were behind the plot (in fact, it was the work of a lone man with far right views).

Sources: The Soufan Group, official estimates from each country. Graphic: Tobey – The Washington Post. Sources: The Soufan Group, official estimates from each country. Graphic: Tobey – The Washington Post.

This new threat, however, seems to show the particular problem posed by the influx of foreign fighters into Syria and Iraq. In an annual assessment on the risks of terror for Norway, the PST estimated that 40-50 people with links to Islamist groups near Oslo had traveled to Syria and had contact with militias there. "Norwegian extreme Islamists who stay with militant Islamist groups in war and conflict areas increase the terrorist threat against Norway and Norwegian interests," the report noted, later adding, "Never before have so many individuals left Norway to fight under militant Islamist groups in conflict areas."

That isn't just a problem that affects Norway, of course. Last November, Thomas Hegghammer of the Norwegian Defense Research Establishment estimated that at least 1,200 European Muslims had traveled to Syria since the civil war started there. "This is a remarkable figure," Hegghammer wrote. "We are talking about the largest European Muslim foreign fighter contingent to any conflict in modern history." Then, to hammer the point home: "In fact, the number of European fighters in Syria may exceed the total number of Muslim foreign fighters from all Western countries to all conflicts between 1990 and 2010."  Other figures suggest that as many as 3,000 foreign fighters may have traveled to Syria.

Hegghammer's research found that Norway's Muslims appeared to be, on a whole, more radicalized, than other European Muslims, though the numbers of Muslims making the trip from places like France and Britain were considerably larger. Some analysts doubt that these foreigners are particularly useful in a battlefield context ("In a context like Isis in Syria you have all these battle-hardy Chechens and then you have some fat guy from Luton turning up," one analyst memorably told the Financial Times) and many of them die, but some make it back. Fifty or more are thought to have returned to Britain, for example, and the British anti-terrorism chief has said it was “almost inevitable” that some would try a home-grown terrorism plot. At least a dozen Americans who have traveled to Syria since the war began have been scrutinized since returning to the U.S., a law enforcement official told The Post last month.

Given the limited information about this alleged Norwegian plot, it's not totally clear how much of a threat it could be, or who exactly is behind it. Norwegian University of Life Sciences terrorism researcher Atle Mesøy told the Norwegian Broadcasting Corporation that it is unlikely al-Qaeda or Islamic State would be directly involved in such a plot, and it more likely involved some kind of smaller Norwegian-linked group. But Norway appears to be taking it very seriously, with Prime Minister Erna Solberg cancelling her summer plans, official buildings closed, and all Norwegian police districts put on alert.