Some defense lawyers and civil rights advocates said the government?s tactics, particularly since the Sept. 11 attacks, have raised questions about the possible entrapment of people who pose no real danger but are enticed into pretend plots at the government?s urging.
But law enforcement officials said on Monday that agents and prosecutors had carefully planned the tactics used in the undercover operation that led to the arrest of the Somali-born teenager, Mohamed Osman Mohamud, 19, a naturalized United States citizen. They said that Mr. Mohamud was given several opportunities to vent his anger in ways that would not be deadly, but that he refused each time.
?I am confident that there is no entrapment here, and no entrapment claim will be found to be successful,? Attorney General Eric H. Holder Jr. said Monday. ?There were, as I said, a number of opportunities that the subject in this matter, the defendant in this matter, was given to retreat, to take a different path. He chose at every step to continue.?
Mr. Holder called the sting operation, in which Mr. Mohamud was under the scrutiny of federal agents for nearly six months, ?part of a forward-leaning way in which the Justice Department, the F.B.I., our law enforcement partners at the state and local level are trying to find people who are bound and determined to harm Americans and American interests around the world.?
A study this year by the Center on Law and Security at New York University, which tracks terrorism cases, found that of 156 prosecutions in what it identified as the most significant 50 cases since 2001, informers were relied on in 97 of them, or 62 percent. The entrapment defense has often been raised, but as of September, it had never been successful in producing an acquittal in a post-Sept. 11 terrorism trial, the study found.
The Portland case resembles several others in which American residents, inspired by militant Web sites, have tried to carry out attacks in the name of the militant Islamic movement only to be captured in a sting operation, with undercover F.B.I. agents or informers playing the role of terrorists and, as in this case, supplying a fake bomb.
In September 2009, Hosam Maher Husein Smadi, a 19-year old Jordanian citizen, was arrested and charged with placing a fake bomb at a Dallas skyscraper. In October, Farooque Ahmed, a 34-year-old naturalized American citizen born in Pakistan, was arrested and charged with plotting to bomb the Washington Metro after meeting with undercover agents and discussing his plans and surveillance activities, the authorities said.
Some Muslim leaders in Oregon questioned how the sting operation there was carried out.
Imtiaz Khan, the president of the Islamic Center of Portland and Masjed As-Saber, a mosque where Mr. Mohamud worshiped, said several people at the mosque had questioned why law enforcement helped orchestrate such an elaborate plan for a terrorist act.
?They?re saying, ?Why allow it to get to this public stunt? To put the community on edge?? ? Mr. Khan said.
Mr. Khan said he and other Muslim leaders met regularly with the F.B.I. and other federal officials. In May, he was among a group of Muslim leaders in the Portland area who issued a statement condemning an attempted bombing in Times Square and thanking law enforcement for its ?outstanding work? in the case.
Jesse Day, a spokesman for the mosque and Islamic center, said the circumstances of Mr. Mohamud?s arrest had stirred ?some distrust, a little bit, in the tactics? of law enforcement.
The government?s 36-page affidavit filed in the Oregon case lays out a crucial conversation between Mr. Mohamud and an F.B.I. informer at their first meeting, on July 30, 2010. According to the affidavit, the informer suggested five ways that Mr. Mohamud could help the cause of Islam, some of which were peaceful, like proselytizing, and some of which were violent and illegal.
Mr. Mohamud, the affidavit said, immediately picked a violent crime: becoming ?operational,? by which he said he meant putting together a car bomb. The informer then offered to put Mr. Mohamud in touch with an explosives expert, setting off the chain of events that led to his eventual arrest.
William Yardley contributed reporting from Portland, Ore., and Scott Shane from Washington.