To stop Internet sex predators, law enforcement officials often pose as teenage girls for men who think they are about to meet them for sex. Those men often travel to an agreed-upon rendezvous point before they are arrested. To prevent murder-for-hire schemes, agents may pose as hit men. Their targets sometimes pose for pictures in which they appear dead as "proof" the hit was carried out.

Drug deals are set up by undercover agents practically on a daily basis.

The use of undercover agents and informants is a staple in everyday law enforcement. Last week's arrest of a Somali teen who hoped to detonate a bomb at a crowded Christmas tree lighting in Oregon shows that terrorism cases are no different. But because it's the latest in a string of recent cases in which undercover agents let a bombing plot play out - ensuring no live bomb is made - criticism is increasing that the FBI is entrapping Muslims who otherwise would not engage in violence.

Mohamed Osman Mohamud, a naturalized American citizen, was arrested by FBI agents Nov. 26 after he thought he had just triggered a massive bomb as thousands of people attended the tree lighting ceremony at Portland's Pioneer Courthouse Square. Mohamud saw the explosives and put the bomb together, but was unaware his suppliers were undercover agents who helped him build a dud.

His defense attorney already promises a defense arguing Mohamud is a victim of government entrapment. Media outlets are taking the claim seriously, with the New York Times citing arguments by a representative whose organization reflexively criticizes law enforcement in reporting operations like Mohamud's undermine trust between American Muslims and law enforcement.

On Tuesday, Attorney General Eric Holder called the investigation "forward leaning" by intercepting someone committed to killing Americans. "'Forward-leaning' seems to be basically if someone has not crossed the bridge, we will push them forward, we will tip them over the edge," Hussam Ayloush, the head of the Council on American-Islamic Relations (CAIR) Los Angeles office told the Times. "And that is not how a government should be treating its citizens."

Similarly, CAIR-San Francisco representative Zahra Billoo told a local television station that the operation was excessive. "The question is, are we looking to stop radicalization and stop extremism before it becomes a problem or do we want a sensational story?" she asked. "And I'd really argue here that the FBI was looking for a sensational story."

CAIR had fewer concerns earlier this year, when FBI undercover agents infiltrated a Christian militia in Michigan, thwarting a plot to kill a law enforcement officer and then ambush the slain officer's funeral. The ultimate goal was to spark a war against the United States government.

In contrast, M. Zuhdi Jasser of the American-Islamic Forum for Democracy took the opposite view, issuing a statement saying the Mohamud case demonstrates the threat radical lone wolf Islamists pose.

"The FBI's tactic of using undercover agents to contact and target Mohamud is more than justified and more than likely saved the lives of thousands of individuals," Jasser said. "Mohamud clearly demonstrated a desire and a will to attack Americans. As we have seen with Nidal Hasan, Faisal Shahzad and Umar Farouk Abdul Mutallab, it is only a matter of time before he would have reached terror leaders such as Imam Anwar Al-Awlaki and secured the means to wreak his destruction. The FBI has a duty to protect our citizens from such attacks and at AIFD we applaud their efforts to disrupt Islamist terrorism."

Initial disclosures in the Portland case indicated Mohamud may be a poor choice on which to build a case against government entrapment. Attorney General Eric Holder got in front of the argument, emphasizing the care agents took to give Mohamud a chance to reconsider. Even Portland's mayor, who helped the city withdraw in 2005 from a Joint Terrorism Task Force out of civil liberties concerns, acknowledges Mohamud was bent on his path to murdering women and children.

"I want whoever is attending that event to leave, to leave either dead or injured," Mohamud told undercover agents Nov. 4. He chose the spot where he wanted to park the car bomb, choosing one "that he thought would inflict the most casualties," an FBI affidavit says.

"This was an individual who was given a number of opportunities to desist from his course of action, but who at every turn decided that he wanted to continue," Holder said Tuesday. "But for the interaction that he had with the FBI, he might have come in contact with somebody who, in fact, would have made his plans tragically real."

Two cases in the past year show that potential terrorists do obtain materials to build bombs without the FBI learning about it. In those cases, disasters were averted due to other problems. A few errors in ingredients and assembly are all that stopped Faisal Shahzad from detonating a car bomb in Times Square earlier this year. FBI agents were able to spook Najibullah Zazi last year as he carried components for a bomb targeting New York subways.

Entrapment arguments rarely succeed and court rulings make clear why Mohamud's case is a weak one on which to make a political stand. In a ruling issued in September, the 9^th Circuit Court of Appeals in San Francisco upheld the conviction of an alleged eco-terrorist who argued a government informant entrapped him. Convicted eco-terrorist Eric McDavid was taped plotting to bomb the Forest Service's Institute of Forest Genetics, the Nimbus Dam and Fish Hatchery, cellular telephone towers, and electric power stations. An informant was with him as he scouted the targeted areas and bought the chemicals to make the bombs.

He appealed his conviction and 20-year prison sentence, arguing the FBI informant entrapped him. The 9^th Circuit rejected that argument in September. "While the jury heard testimony from McDavid's sister and friend about McDavid's peaceful and gentle nature," the court ruled, "the testimony was overwhelmed by more specific evidence that McDavid had become radicalized, believed that nonviolent protests were ineffective, and was undaunted by the possibility of accidental deaths from his actions."

To prove entrapment, a defendant must show he would not have pursued the crime if left on his own. In McDavid's case, the 9^th Circuit ruled, "the evidence showed that McDavid suggested targeting the IFG, and the group discussed and researched the IFG extensively. McDavid also initiated the idea of using explosives, and the group discussed bomb types, bought materials, and attempted to make an explosive."

In the Portland case, the FBI claims Mohamud was radicalized on his own, saying he had wanted to carry out a jihadist attack since he was 15. He initiated the idea of using explosives, the complaint against him alleges, and he persisted after undercover agents cautioned him about the potential results.

Mohamud knew what he wanted. The FBI made sure that never happened.