The alleged plot to detonate a car bomb duringPortland's tree lighting celebrationfollows a string of terrorist attack planning by U.S. citizens or residents.

In the Times Square plot, Faisal Shahzad allegedly tried to set off a car bomb at a bustling street corner. U.S. authorities had no intelligence about Shahzad's plot until the smoking car turned up in Manhattan.

Late last month, Farooque Ahmed, 34, of Virginia was arrested and accused of casing Washington-area subway stations in what he thought was an al-Qaida plot to bomb and kill commuters. Similar to the Portland sting, the bombing plot was a ruse conducted over the past six months by federal officials.

A year ago in another federal sting, 19-year-old Jordanian Hosam Smadi was arrested on charges he intended to bomb a downtown Dallas skyscraper. Federal officials said he placed what he believed was a car bomb outside the building but was instead a decoy device given him by an undercover FBI agent.

I think we've been extremely lucky so far in the United States that many of the incidents have been amateur, said Bruce Hoffman, terrorism expert at Georgetown University. But even if their skill level is not enough that they can pull off a successful attack, what is clear that the intention or motivation to cause mass homicide or destruction is certainly genuine.

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