US law enforcement agencies have thwarted an attempt to blow up a car bomb at an annual Christmas tree lighting ceremony on Friday evening in Portland, Oregon, officials said early on Saturday.
The Justice Department identified the chief suspect as 19-year-old Mohamed Osman Mohamud of the town of Corvallis, a naturalised US citizen of Somali descent.
Mohamud was arrested by the Federal Bureau of Investigation and Portland police around 5:40pm on Friday (0040 GMT Saturday) after he attempted to detonate what he believed to be an explosives-laden van that was parked near the tree lighting ceremony in Portland's Pioneer Courthouse Square, officials said.
But the device was in fact inert, and the public was never in any danger, they added.
"The threat was very real. Our investigation shows that Mohamud was absolutely committed to carrying out an attack on a very grand scale," said Arthur Balizan, a special agent in charge of the FBI in Oregon. "At the same time, I want to reassure the people of this community that, at every turn, we denied him the ability to actually carry out the attack."
According to court documents, in August 2009, Mohamud was in email contact with an accomplice overseas who is believed to be involved in terrorist activities.
In December 2009, while the accomplice was located in the north-west frontier province of Pakistan, Mohamud and the suspect discussed the possibility of Mohamud traveling to Pakistan to engage in jihad.
This man allegedly referred Mohamud to a second conspirator overseas and provided Mohamud with a name and email address to facilitate the plot, according to the court documents.
In the months that followed, Mohamud allegedly made several unsuccessful attempts to contact his partner.
Ultimately, last June, an FBI undercover operative contacted Mohamud via email under the guise of being an associate of his Pakistani contact.
Mohamud and the FBI undercover operative then agreed to meet in Portland in July.
At this meeting, Mohamud told the FBI undercover operative that he had written articles that were published in "Jihad Recollections," an online magazine that advocated violence against non-Muslims.
He later told undercover FBI operatives that he had been thinking of conducting a holy war against infidels since the age of 15 and that he had identified a potential target for a bomb: the annual Christmas tree lighting ceremony in Portland's Pioneer Courthouse Square on November 26, 2010, the documents allege.
FBI operatives cautioned Mohamud several times about the seriousness of this plan, noting that there would be many children at the event. But Mohamud responded that he was looking for a "huge mass that will ... be attacked in their own element with their families celebrating the holidays," the affidavit pointed out.