Meanwhile, three Iraqi journalists were killed this morning when men in a car opened fire on their minibus in the city of Baquba. The journalists worked for a coalition-funded television station, Diyala TV.
The bombing and the murders of the journalists are two of the latest in a string of attacks aimed at foreign civilians working in the country, as well as Iraqis seen to be cooperating with the coalition.
"This is a terrible terrorist attack on innocent civilians. Our thoughts and prayers are with the victims and with the families. This remains a time of testing in Iraq. The stakes are high, the terrorists know the stakes are high, but they will not prevail," McClellan said.
A German and a Dutch contractor were killed 16 March in a drive-by shooting near the town of Mussayab, some 70 kilometers south of Baghdad.
The day before, five American missionaries affiliated with the Southern Baptist denomination were killed in an ambush in the northern city of Mosul. The missionaries had just arrived in Mosul to help build a water-purification plant for refugees.
Several days earlier, two American civilians working for the coalition and their Iraqi interpreter were killed at a false checkpoint near the town of Hilla, south of Baghdad.
Previous attacks have targeted Iraqi police. At least 350 Iraqi police officers and security personnel have died in such attacks since May -- more than 100 last month alone.
The commander of U.S. forces in Iraq, General Ricardo Sanchez, said this week that the attacks "clearly [show] there has been a shift in the insurgency and the way the extremists are conducting operations." He said the attacks are an attempt to intimidate those who are trying to help the U.S.-led coalition.
Baseem Sadoun works for International Medical Corps, a U.S. nonprofit humanitarian relief organization that is working in Iraq. He says the attacks are frightening his international staff. He says they are spending more time inside their compounds and trying to cut their travel around the country.
Sadoun says Westerners in Iraq feel particularly vulnerable.
"To the fanatics or the Islamists -- I don't know who is doing these things -- you only have to [look] different [from the crowd] and you are attacked because they don't know if you are working with the Americans or if you are not working with the Americans. You are a foreigner, and they don't care. They just attack," Sadoun said.
Mustafa Alani is an associate fellow in the Middle Eastern program at the Royal United Services Institute for Defense Studies in London. He says Iraqi civilians and foreigners are being increasingly targeted for attacks in Iraq not only because they are "soft" targets -- that is, they have little, if any, security protection.
More importantly, he says, the attacks may indicate a change in the political agenda of the Iraqi resistance.
"[They] want to isolate the Americans in Iraq. [They] want to put pressure on any party -- whether a state or an organization -- [and make it] run from Iraq and to isolate the Americans in Iraq -- to leave them alone in Iraq," Alani said.
He says the increase in attacks may mark only the beginning of a trend. He believes the attacks could have serious consequences for the U.S. occupation.
"At the end of the day, democratic [countries] will not send their children to die when they know there is a high risk because they have political responsibility. Germany or other nations will think twice before sending their volunteers, or even their army, in the future. So, certainly, this will have an effect on the psychology of their people. [People will think] that Iraq is a very dangerous place and will not commit themselves to this sort of place," Alani said.
Alani notes that the attacks last year on the Red Cross headquarters in Baghdad and on the United Nations mission in the capital forced those groups to withdraw much of their staff from the country.
Paul Wilkinson is the head of the Center for the Study of Terrorism and Political Violence at St. Andrew's University in Scotland. He sees no principal differences between the methods used in the terrorist attacks of 11 September 2001, last week's train bombings in Madrid, or the attacks against unarmed civilians working on reconstruction projects in Iraq. All such attacks, he says, are difficult to prevent using traditional military force.
"[Terrorism] is a method that is used by people who are dressed just as you and I are, who can merge in with the civilian population, who live in the cities around the world undercover apparently -- [living] perfectly respectable lives, perhaps as students or businessmen or whatever. And the fight against their kind of terrorism has to be conducted with a multipronged strategy, which has to be intelligence-led," Wilkinson said.
"The Washington Post" quotes senior U.S. intelligence officials as saying that the CIA has deployed four times as many clandestine officers in Iraq as it had planned on, but admitted it has had little success penetrating the resistance.
Meanwhile, there are many foreign civilians working in Iraq, and their numbers are likely to grow if reconstruction efforts continue as planned.
"The New York Times" reports that -- counting members of the occupation government, journalists, contractors, aid workers, and others -- there are probably more than 5,000 foreign civilians in Iraq. Some Western companies are paying bodyguards $1,000 a day to protect their workers.
"And with $18 billion of reconstruction projects just beginning," "The New York Times" says, "the demand will only increase."