University of Chicago professor Robert Pape has compiled an intensive study of suicide terrorism. He says more suicide attacks can be expected as the terrorists become increasingly frustrated over the impotence of head-on military action.

"As we have seen over the last 20 years, suicide terrorist attacks tend to come in clusters of well-organized campaigns, and happen when terrorists are trying to end a military occupation, and come to believe that other military tactics will not work. And as we have seen in Lebanon, Israel, Chechnya, and numerous other places, once suicide campaigns start, they tend to go on for a long time.

President Bush blames supporters of deposed president Saddam Hussein and people from outside Iraq for the suicide attacks. "We are trying to determine the nature of who these people were. But I will tell you, I would assume that they are either, or, and, probably, both Baathists and foreign terrorists," he said.

Analysts believe the occupation of Iraq has drawn anti-U.S. zealots to the country seeking to attack U.S. interests. The foreigners and the Saddam loyalists have found common cause in their hatred of the United States.

Mr. Pape says the exact mix of local and outside support is hard to determine. "We can not yet say exactly what the balance is between local support and external support," he said. "But I think it is fair to say that it is likely to be a mixture of both. And we are likely to discover that the internal support is quite significant."

The attackers also seem to have widened their circle of targets. They once hit only U.S. forces, but they are now striking at any foreign presence, no matter how benign.

Most significantly, says Mr. Pape, they are attacking Iraqis who they believe to be collaborators with the U.S. occupation. "I think it shows that they are trying to inflict costs not only on Americans, but on the local Iraqis who are cooperating with Americans. And I think that they believe that if they inflict heavy costs, there will be fewer and fewer willing to cooperate with Americans," he said.