Mohamed Mohamud, 19, a U.S. citizen born in Somalia, was arrested in Portland Friday night as he thought he was detonating a car bomb with his cellphone in a packed Pioneer Courthouse Square, authorities said.
Although the bomb was fake, "the threat was very real," FBI agent Arthur Balizan said in a statement reported by The New York Times.
Mohamud was charged with trying to use a weapon of mass destruction.
In contrast to the attempted Times Square bombing last spring, which was discovered at the last minute, the FBI had been following Mohamud since 2009, and his plot unfolded under the scrutiny and with the aid of undercover agents, Balizan said.
The surveillance began in August 2009 after agents intercepted Mohamud's e-mails with a terrorist recruiter who had returned from Oregon to the Middle East, according to a law enforcement affidavit.
In June, after Mohamud tried to go overseas for jihadist training, an undercover agent contacted him. On July 30, the FBI first met him in person to initiate the sting operation.
Mohamud wanted to kill as many people as possible and chose the Christmas target, the FBI said.