“Mo-Mo,” his friends called the slim young Oregon State University student who parked a white van filled with what he thought were explosives at a crowded Christmas tree-lighting in Portland last year.

Federal Bureau of Investigations agents arrested Mohamed-Mohamed Mohamud as he repeatedly dialled the cellphone that should have triggered a devastating blast in the midst of the 25,000-strong crowd — if the explosives hadn't in fact been harmless putty, planted by undercover officers.

“You know what I like to see,” court documents show Somalia-born Mohamud asking one undercover operative? “Is when I see the enemy of Allah then, you know, their bodies are torn everywhere [sic.].”

The New York Times, he said of the planned bombing, “will give it two thumbs up.”

For many in the United States, the unfolding trial of Tahawwur Rana, a Pakistani-Canadian businessman who prosecutors allege aided the 26/11 attacks, represents fresh evidence of a growing threat to their homeland.

Mr. Rana is just one of at least 180 residents and citizens of the U.S. held since the tragic events 9/11 on charges of aiding Islamist terror groups. The number of jihadist terrorism cases involving residents of the U.S. has been rising steadily: there were 33 arrests in 2010, and another 43 in 2009 — together, almost half the number held in all years since 2001.

Last year, the Federal Bureau of Investigations arrested Pakistani-origin taxi driver Raja Lahrasib Khan. Mr. Khan is alleged to have plotted with Ilyas Kashmiri — Mr. Headley's mentor, who was reported killed in a drone strike over the weekend — to blow up a stadium in Chicago.

In a video released online on June 3, Adam Pearlman — the 1978 born, California-raised jihadist who became known to the world as al-Qaeda propaganda head Adam Gadahn — called for more such operations. “Muslims in the West,” he said, “have to remember that they are perfectly placed to play an important and decisive part in the jihad against the Zionists and the Crusaders.”

The American jihadists

The American jihadists are a diverse crew: a study released this year by the Washington, DC-based New America Foundation showed 24% were of West Asian descent, 21% Caucasian, and 18% each Somali or Pakistani. Some were young, others old; some poor, some rich.

Few were archetypical jihadists. “He hung out with people who partied and drank alcohol,” recalled Omar Mohamed, who studied with Portland's Mohamud, in an interview to the local radio station KVAL. “He didn't commonly go to the mosque. He wasn't an obedient Muslim.”

It remains unclear just what led Mr. Mohamud, just 19 at the time of his arrest, to embrace Islamism. His father, Osman Barre, an engineer at Intel, is reported to have contacted the FBI to express his growing concern about his son's politics.

Friends of Raeed Mansour al-Banna, who blew himself up outside a medical clinic in the Iraqi town of Hillah in 2005, also had memories of an unorthodox man. “We hit some pretty wild clubs in Hollywood,” recalled Steve Gray, who worked with al-Banna at Ontario International Airport.

Pearlman's own case is instructive. Born to the musician Phil Pearlman, the al-Qaeda public relations chief began attending an Islamist study circle as a teenager, to address what he described as a “yawning emptiness.” The grandson of Carl Pearlman, a zealous Zionist, Mr. Pearlman experimented with Christian neo-fundamentalism before travelling to Pakistan, and joining al-Qaeda.

Even eccentrics have been drawn to the American jihad: members of a group called the ‘Liberty City Seven' belonged to a cult called the Seas of David, which had splintered away from the fringe Moorish Science Temple of America, which purports to combine the teachings of Christianity and Islam.

One of the Liberty Seven made a living practising voodoo.

Fears aside, few plots so far have actually come to fruition: bar Nidal Malik Hasan, a soldier who succeeded in killing 13 when he opened fire inside the Fort Hood base in Texas, America's would-be jihadists have mostly been held before they could do harm.

Faisal Shahzad, of Pakistani origin, hoped to kill dozens at Times Square in New York last year, but the car bombs he built failed to operate. Najibullah Zazi, an Afghan, was arrested before he finished assembling improvised explosive devices.

More than a few experts believe fears of a large-scale jihadist offensive by Muslims living in the United States are overstated. The 1970s, Brians Jenkins pointed out in a thoughtful study published last year, “saw 60 to 70 terrorist incidents, most of them bombings, on U.S. soil every year — a level of terrorist activity 15 to 20 times more than seen in most of the years since 9/11.”

From 1970 to 1978, he continued, “72 people died in terrorist incidents, more than five times the number killed by jihadist terrorists in the United States in the almost nine years since 9/11.”

“In Europe and elsewhere,” says Daveed Gartenstein Ross, “the Islamist movement is rooted in wider conflicts between Muslim immigrants and the societies they live in. In the U.S., those conditions do not exist in the same way — study after study, after all, has shown that Muslims are wealthier than average.”

Even the small scale and limited success of the American jihadist movement has raised deep fears, though, of what could lie ahead: a sign that even the death of Osama bin Laden has done little to heal the scars of a country still haunted by the memories of 9/11.

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