Just before a 22-year-old Florida man blew himself up in Syria last May, he made one last trip home.
US authorities weren't tracking him. They didn't learn that Moner Mohammad Abusalha had become radicalised until shortly before his suicide attack, said two US officials who spoke on condition of anonymity because they were not authorised to be quoted discussing sensitive intelligence.
Abusalha, who grew up in Florida, is part of what officials call a disquieting and longstanding trend in Western extremism that seems also to include last week's Paris attacks.
The links to al Qaeda run a gamut, analysts say, from the disturbed Muslim convert in Oklahoma who beheaded a former co-worker at a meat-packing plant in September, to the ideologically committed brothers in Paris who attacked the satirical Charlie Hebdo newspaper. The Oklahoma man had no connection to any terror group, while brothers Cherif and Said Kouachi are believed to have consulted with al Qaeda's affiliate in Yemen.
On Wednesday, an Ohio man was arrested and charged with plotting to kill government officials inside the US Capitol. The FBI said the man spoke of his desire to support the Islamic State group.
Other such cases in the US include the 2013 Boston marathon bombings, the 2010 attempted Times Square bombing and the 2009 Fort Hood shootings. In Europe, an investigation found no direct assistance or orchestration from al-Qaida to the group that bombed a Madrid train in 2004. Likewise, authorities have found no links between international terrorists and the man who attacked Canada's parliament in October.
Homegrown, less well-coordinated and more self-contained than a massive attack like September 11, these plots involve fewer communications for intelligence agencies to intercept, fewer potential sources to turn. "Lone wolves," such as Abusalha, a Palestinian American who decided to wage Jihad, are the most difficult to unearth, officials say.
"You have individuals who are inspired by the ideology but aren't directly connected to any specific group," said John Cohen, a former Department of Homeland Security counterterrorism coordinator who is now a professor at Rutgers University. "They are very difficult for our traditional counterterrorism capabilities to pick up on."
French authorities were aware of the extremists who carried out last week's attacks in France, but the men had not been perceived as significant threats by the overwhelmed domestic intelligence service. More than 1,000 extremists have left France to fight in Syria and some have returned home.
Cherif Kouachi said he met in Yemen with Anwar Awlaki, the American cleric who was killed in Yemen in 2011 in a CIA drone strike. Don Borelli, a former senior FBI counterterrorism official, said the Paris case seems to fit a pattern: "They will go and get indoctrinated, learn all the skills, they'll get support. But then they do their own reconnaissance of the target and pick the time that's right."
That terror template, Borelli said, has evolved in part because of successful surveillance by the US National Security Agency, which has been effective in intercepting conversations between terrorists overseas.
Borelli helped make the case against Najibullah Zazi, the Afghan-American who pleaded guilty to spearheading a September 2009 plot against the New York City subway system, in part through NSA intercepts of Zazi's email communications with a Pakistani based terrorist.
"The intelligence community is a lot more tuned-in to picking up on those communications that are necessary to have it commanded and controlled overseas," Borelli said.
American Muslims are far more integrated into society than their counterparts in Europe, so there are fewer opportunities for disaffection and radicalization, experts say. At the same time, US free speech and civil liberties guarantees set a high bar for surveillance of potential extremists. The FBI looked into the Boston Marathon bombers but closed the case after finding no basis to proceed.