PORTLAND, Ore. — For more than two years, the only image the public has had of the man accused of plotting to detonate an 1,800-pound bomb at a Portland Christmas tree-lighting ceremony is this: A sullen-faced, sunken-eyed terrorism suspect in a mug shot taken just hours after his arrest.

At the trial that begins Thursday, Mohamed Mohamud’s attorneys will attempt to present a different image, one of an impressionable teenager lured by undercover agents with the FBI, which snared one of its youngest terrorism suspects with his arrest in November 2010.

At issue is whether Mohamud was entrapped, as his defense claims, when he gave the go-ahead for the detonation of what he thought was a bomb at the Christmas tree-lighting ceremony. The bomb was a fake, provided by FBI agents whom the 19-year-old thought were his jihadist co-conspirators.

It was one in a series of high-profile FBI terror stings dating back to the Justice Department’s directive to ramp up its terror prosecutions and informant network after the 9/11 terror attacks.

Based on pretrial filings, one of the avenues Mohamud’s attorneys are likely to pursue is based on an undisputed fact: Mohamud was a teenager when he was arrested, and his attorneys allege he was still a minor when the FBI began to focus on him.

This, his attorneys say, made him much more vulnerable to FBI enticements, and a jury should consider him an unwilling pawn of a Justice Department hungry for a conviction that demonstrates its regard for terrorism as its highest priority.

This, too, is not in dispute: Mohamud pushed a button on a cellphone that he thought would set off a bomb placed in a van and kill thousands.

The FBI alleges in court documents — and backed it up with transcripts of conversations secretly recorded by undercover agents — that Mohamud picked the time and place of the detonation. The high school graduate from Beaverton, Ore., knew the area and knew that the event would be well-attended.

“It’s gonna be a fireworks show,” the FBI says he told undercover agents. “A spectacular show.”

Prosecutors also allege Mohamud “explained how he had been thinking of committing some form of violent jihad since the age of 15,” according to the affidavit filed in connection with his arrest.

Mohamud’s attorneys have a high bar to cross, said Karen Greenberg, director of the Center on National Security at Fordham Law School.

“The entrapment defense is really difficult, much more difficult when it comes to terrorism cases,” Greenberg said.

Juries are being asked to weigh heavy legal questions of predisposition against more visceral evidence like secret audio recordings of the defendant praising violent jihad. “Once you’re accused of terrorism (in front of U.S. juries), you’re presumed to be guilty,” Greenberg said.

Attorneys from both sides are forbidden from speaking about the case publicly.

For a time, Mohamud was able to live two lives — as a young immigrant trying to fit in, and a Muslim who had become radicalized.

Mohamed Mohamud’s family emigrated from Mogadishu, Somalia, where he was born in 1991. He moved to the U.S. when he was about 5 years old.

Mohamud professed aspirations of becoming an engineer, like his father. As a student at Oregon State University, he spent his freshman year studying, playing basketball and partying but eventually dropped out.

As a senior in high school, Mohamud had begun writing articles for an online English-language jihadist magazine called “Jihad Recollections” under the pen name Ibn al-Mubarak, advocating physical fitness for the mujahedeen in places where they couldn’t find exercise equipment.

He wrote three articles, including one praising the content and presentation of al-Qaeda’s media arm, As-Shabab Media.

The FBI began monitoring Mohamud’s emails. In the summer of 2010 FBI undercover agents set up the first in a series of meetings with Mohamud, who talked about a dream in which he led a group of fighters into Afghanistan against “the infidels.”

According to the prosecution’s version of events, Mohamud’s undercover handlers offered him several choices in the service of jihad. They ranged from simple prayer to full-on martyrdom. Mohamud chose a step short of killing himself, saying he wanted to “become operational,” according to the FBI.

This, they say, should show that Mohamud was more than an unwitting teenager.

Journalist Trevor Aaronson found a common thread in such sting cases, documented in a forthcoming book, “The Terror Factory: Inside the FBI’s Manufactured War on Terrorism,” which spends a chapter on elements of the Mohamud case.

“(The stings) all have minor variations, but they’re all pretty much the same in that they involve people who don’t have the capacity to commit the crimes” for which they’re prosecuted, Aaronson said.

Aaronson said Mohamud didn’t have access to bomb-making materials and, while he espoused anti-Western views, showed no capacity for carrying out acts of terror.

“If you’re going to prosecute every loudmouth,” Aaronson said, “our courts would be clogged.”