A Somali native faces life in prison after being convicted Thursday of trying to detonate a car bomb at a crowded 2010 tree lighting ceremony in Portland, Ore.

Mohamed Mohamud's attorneys argued that the 21-year-old was entrapped by FBI agents and otherwise would not have pursued such a plot. Jurors paid attention to that argument, asking the judge during deliberations for a written definition of entrapment "so we can keep it in mind while we listen to the deliberations," court records show.

But evidence presented during the three-week trial showed Mohamud was determined to engage in a terrorist attack without any prodding. The law requires that prosecutors prove Mohamud was "predisposed" to plot an attack before FBI informants or undercover agents contacted him, or that the agents were not the ones who persuaded him to get involved in criminal activity.

Mohamud was arrested inside a parked car as thousands of people gathered in Portland's Pioneer Courthouse Square for an annual Christmas tree lighting. He pressed buttons he thought would trigger an 1,800-pound truck bomb nearer to the crowd. But FBI agents controlled the bomb's construction and ensured it was inert.

FBI agents were drawn to him after Mohamud wrote four articles for Jihad Recollections, an online magazine, and posted on other jihadi web sites. In addition, before he tried repeatedly to go to Yemen for training in jihad, prosecutors said.

And the bombing wasn't their idea. During his first meeting with an FBI undercover agent, Mohamud rejected non-violent ideas, choosing instead to serve "the cause of radical, violent Islamic jihad," prosecutors said. Mohamud said that he wanted to "wage war." He told the undercover he wanted to be "operational," explaining he wanted to build a bomb.

Islamist activists and others have criticized sting operations like the Mohamud case, accusing the FBI of manufacturing terrorists who otherwise would pose no risk. That defense has not been persuasive in court.

In a statement released after the verdict, Greg Fowler, the FBI's Special Agent in Charge of the Portland office, said the case shows the challenges of keeping people safe from homegrown terrorists.

"Mr. Mohamud made a series of choices over a period of several years—choices that were leading him down a path that would have ended in violence," Fowlers said. "His actions showed little regard for the rights and responsibilities that come with being an American or respect for the lives that he was prepared to take."