PORTLAND, Ore. - A Somali-born teenager plotted "a spectacular show" of terrorism for months, saying he didn't mind that children would die if he bombed a crowded Christmas tree-lighting ceremony, according to a law-enforcement official and court documents.

He never got the chance. Mohamed Osman Mohamud, 19, was arrested Friday in downtown Portland after using a cell phone to try to detonate what he thought were explosives in a van, prosecutors said. It turned out to be a dummy bomb put together by FBI agents.

It is the latest in a string of alleged terrorist plots by U.S. citizens or residents, including a Times Square plot in which a Pakistan-born man pleaded guilty earlier this year to trying to set off a car bomb at a bustling street corner. Last month, another Pakistan-born Virginia resident was accused in a bomb plot to kill commuters.

In the Portland plot, Mohamud believed he was receiving help from a larger ring of jihadists as he communicated with undercover federal agents, but no foreign terrorist organization was directing him, according to a law-enforcement official who wasn't authorized to discuss the case publicly and spoke on a condition of anonymity.

The official said Mohamud was very committed to the plot and planned the details alone, including where to park the van to hurt the most people.

"I want whoever is attending that event to leave, to leave dead or injured," Mohamud said, according to the affidavit.

"It's in Oregon, and Oregon, like you know, nobody ever thinks about it," the suspect told an agent in one discussion.

Thousands of people had gathered Friday on a cold, clear night for the annual event at Pioneer Courthouse Square, a plaza often referred to as "Portland's living room" because of its popularity.

Just 10 minutes before Mohamud's 5:40 p.m. arrest, the lighting ceremony began. Babies sat on shoulders, and children cheered at the first appearance of Santa Claus onstage.

The tree-lighting on the bricks of the plaza went off without a hitch just as the arrest was taking place.

Mohamud, who grew up in Beaverton, was a former student at Oregon State University. He had been enrolled in courses from late 2009 until Oct. 6 before withdrawing, said Oregon State University spokesman Todd Simmons.

The law-enforcement official who spoke to the AP on Saturday said agents began investigating Mohamud after receiving a tip from someone who was concerned about the teenager. The official declined to provide any more detail about the relationship between Mohamud and that source.

In an e-mail exchange with an undercover agent Mohamud complained, "I have been betrayed by my family," although he describes no specific action that family members took.

The FBI monitored Mohamud's e-mail and found that he was in contact with people overseas, asking how he could travel to Pakistan and join the fight for jihad, according to an FBI affidavit.

According to the law enforcement official, Mohamud e-mailed a friend living in Pakistan who had been a student in Oregon in 2007-2008 and been in Yemen as well.

For reasons that have not been explained, Mohamud tried to board a flight to Kodiak, Alaska, from Portland on June 14, wasn't allowed to board and was interviewed by the FBI, the affidavit states.

Mohamud told the FBI he wanted to earn money fishing and then travel to join "the brothers." He said he had previously hoped to travel to Yemen but had never obtained a ticket or a visa.

On June 23, an undercover agent contacted Mohamud by e-mail, pretending to be affiliated with the "unindicted associate" Mohamud had sent e-mails to.

The FBI's affidavit says the friend in Pakistan referred him to another associate, but gave him an e-mail address that Mohamud tried repeatedly to use unsuccessfully. The official said FBI agents saw that as an opportunity and e-mailed Mohamud in response, claiming to be associates of his friend, the former student.

The affidavit said Mohamud was warned several times about the seriousness of his plan, that women and children could be killed, and that he could back out. But he told agents: "Since I was 15 I thought about all this" and "It's gonna be a fireworks show ... a spectacular show."

Mohamud, a naturalized U.S. citizen living in Corvallis, was charged with attempted use of a weapon of mass destruction, which carries a maximum sentence of life in prison. A court appearance was set for Monday.

Authorities allowed the plot to proceed in order to build up enough evidence to charge the suspect with attempt. Mohamud sent bomb components to undercover FBI agents who he believed were assembling the explosive device, but the agents supplied the fake bomb that Mohamud tried to detonate twice via his phone, authorities said.

The FBI affidavit says the undercover agent first met Mohamud in person on July 30 and asked what he would do for the cause of jihad. The agent suggested that Mohamud might want to spread Islam to others, continue his studies to help the cause overseas, raise money, and become a martyr or put together an explosion but didn't know how and needed training, the affidavit said.

The undercover agent said he could introduce him to an explosives expert and asked Mohamud to research potential targets.

At a second meeting on Aug. 19 at a Portland hotel, the agent brought a second undercover agent, the documents say, and Mohamud told them he had selected Portland's Pioneer Courthouse Square for the bombing.

On Nov. 4, the court documents say, Mohamud made a video in the presence of one of the undercover agents, putting on clothes he described as "Sheik Osama style:" a white robe, red and white headdress, and camouflage jacket.

He read a statement speaking of his dream of bringing "a dark day" on Americans and blaming his family for thwarting him, according to the court documents:

"To my parents who held me back from Jihad in the cause of Allah. I say to them ... if you - if you make allies with the enemy, then Allah's power ... will ask you about that on the day of judgment, and nothing that you do can hold me back ..."

Friday, an agent and Mohamud drove to downtown Portland in a white van that carried six 55-gallon drums with detonation cords and plastic caps, but all of them were inert, the complaint states.

They left the van near the downtown ceremony site and went to a train station where Mohamud was given a cell phone that he thought would blow up the vehicle, according to the complaint. There was no detonation when he dialed, and when he tried again federal agents and police made their move.

Tens of thousands of Somalis have resettled in the United States since their country plunged into lawlessness in 1991, and the U.S. has boosted aid to the country.

In August, the U.S. Justice Department unsealed an indictment naming 14 people accused of being a deadly pipeline routing money and fighters from the U.S. to al-Shabab, an al-Qaida affiliated group in Mohamud's native Somalia.

Officials have been working with Muslim community leaders across the United States, particularly in Somali diasporas in Minnesota, trying to combat the radicalization.

On Saturday, Omar Jamal, first secretary to the Somali mission to the United Nation and an advocate for Somalis in Minnesota, said Mohamud has a stepmother in Minneapolis. He condemned the plot and urged Somalis to cooperate with police and the FBI.

Jamal said he had spoken to two Somalis who knew Mohamud, and he was described as religious, quiet and innocent. Jamal said Mohamud is from southern Somalia.

"Everybody's afraid, really really afraid," Jamal said of members of Oregon's Somali communities and elsewhere. "They're afraid of, first of all, the label. The allegation is very serious ..."