A Somali-American was found guilty Thursday over an alleged plot to blow up a Christmas tree lighting ceremony in the northwestern US state of Oregon more than two years ago.
The jury returned its verdict on Mohamed Mohamud less than a day after retiring to deliberate after his trial over the attempted attack on November 26, 2010.
Lawyers for the 21-year-old, who faces up to life in prison for plotting to use a weapon of mass destruction, claimed he was tricked into the attempted bombing by undercover agents from the Federal Bureau of Investigation.
But Oregon prosecutors insisted during the nearly three-week trial that the young man actively participated in a plot that could have killed thousands, had the bomb been real.
"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," Greg Fowler, head of the FBI office in Portland, said after the verdict.
"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," he added in a statement.
Sentencing was set for May 14 at the federal US District Court in Portland, near the scene of the crime.
Mohamud's lawyers planned to appeal the verdict, according to The Oregonian newspaper.
In closing arguments Wednesday, defense lawyer Stephen Sady said Mohamud, who was first contacted by an undercover agent a year before the attempted bombing, was a troubled teenager led on by the FBI.
"He's not a threat," the publicly-appointed attorney said, cited by local ABC television affiliate KATU. "He's simply a person trying to live through a difficult adolescence.
"In America, the government cannot create a crime," he added.
But Assistant US Attorney Ethan Knight dismissed the claim, saying: "We are not talking about an adolescent period... We're talking about a bomb."
On Thursday, FBI agent Fowler said the guilty verdict "highlights the difficult but important work that FBI employees do every day."
"In this country, everyone has a right to live, work and worship freely and without fear. FBI employees -- in Oregon and around the world -- find strength in preserving and protecting these core values," he said.
Fowler thanked the jurors, saying: "I know that they carried a heavy burden -- deciding the fate of a young man while balancing the needs for safety and justice.
"We greatly appreciate their service to their country."
At the start of the trial, an FBI agent who gave evidence in disguise maintained that Mohamud was prone to violence from the beginning and spoke of plans to "wage war" on the United States.
To test Mohamud's resolve, the agent -- who was posing as an Al-Qaeda recruiter named "Youssef" -- said that in his first meeting with Mohamud, he gave the then-teenager five examples of how he could be "a good Muslim."
According to Youssef -- whose real name was withheld in court -- Mohamud stopped short of the most extreme option, martyrdom, but chose violence over praying five times a day or raising money for militant groups.
The defense argued that FBI agents such as Youssef used powerful psychological tools to brainwash a confused teenager, giving him specific instructions on how to plan an attack he wasn't capable of on his own.
Under US law, it is illegal for authorities to trick someone into committing a crime.