Federal law enforcement official thwarted an attempt Friday by a Somalia-born U.S. citizen to explode a bomb during a crowded Christmas tree-lighting ceremony in Portland, Ore., the FBI announced.

Mohamed Osman Mohamud, 19, of Corvallis, Ore., was arrested at about 5:40 p.m. PST after he attempted to detonate a van loaded with explosives in Portland's Pioneer Courthouse Square. However, undercover FBI agents had been working with Mohamud during his months-long plot, and the explosives were inert, the FBI said.

Mohamud is expected to appear in federal court for the first time on Monday. He faces a maximum sentence of life in prison and a $250,000 fine if convicted of the charge of attempting to use a weapon of mass destruction, the FBI said.

While Mohamud is from Somalia, the FBI affidavit said he was first noticed after he tried to contact an unknown associate in Pakistan by email in August 2009. Mohamud originally planned to travel to Pakistan to wage jihad, the FBI said, but those plans didn't pan out. In June, an undercover FBI operative contacted Mohamud in the guise of being an associate of the unknown Pakistani contact.

"At this meeting, Mohamud allegedly told the FBI undercover operative that he had written articles that were published in Jihad Recollections, an online magazine that advocated violent jihad," the FBI said. "Mohamud also indicated that he wanted to become 'operational.' Asked what he meant by 'operational,' Mohamud stated that he wanted to put an 'explosion' together, but needed help."

Friday's arrest is one in a recent string of cases involving homegrown terrorism plots. In October, Pakistan-born U.S. citizen Farooque Ahmed was arrested for plotting to blow up the Washington, D.C., Metro system. In May, Pakistan-born U.S. citizen Faisal Shahzad was arrested for trying to blow a car bomb in New York's Times Square. He was sentenced to life in prison in October.