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WASHINGTON — U.S. and French intelligence officials are leaning toward an assessment that the Paris terror attacks were inspired by al Qaeda but not directly supervised by the group, a view that would put the violence in a category of homegrown incidents that are extremely difficult to detect and thwart.

Although one of the two brothers who carried out the attack at the Charlie Hebdo newspaper is believed to have traveled briefly to Yemen in 2011, where he met an al Qaeda leader, U.S. intelligence officials are not convinced that the Paris attacks were directed from abroad, despite a claim of responsibility by al Qaeda's Yemen affiliate. The claim seems hastily put together and "opportunistic," as two senior officials put it, one French and the other American, both declining to be named in order to discuss sensitive intelligence.

Investigators also are not convinced that Amedy Coulibaly, who killed five people in Paris in separate incidents, coordinated in advance with Cherif and Said Kouachi, who killed 12 in the attack on the Charlie Hebdo newspaper.

If those assessments hold, they would place the attacks on a continuum of violence by disaffected individuals who have become sympathetic to al Qaeda, the Islamic State group or their ilk — yet are not involved in the sort of international conspiracy that lends itself to relatively easy detection.

The cases 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 newspaper. The Oklahoma man had no connection to any terror group, while the Kouachi brothers 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 U.S. Capitol. The FBI said the man spoke of his desire to support the Islamic State group.

Other such cases in the U.S. 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 Qaeda 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.

Home-grown, less well-coordinated and more self-contained than a complex attack like those of Sept. 11, these plots involve fewer communications for intelligence agencies to intercept, fewer potential sources to turn. "Lone wolves," such as Moner Mohammad Abusalha, a Palestinian-American who decided to wage jihad, are the most difficult to unearth, officials say.

In the U.S., the FBI wasn't aware that Abusalha, a Vero Beach, Florida, resident, had become radicalized until shortly before he launched a suicide attack in Syria in May, said two U.S. officials who spoke on condition of anonymity because they were not authorized to be quoted discussing sensitive intelligence.

"You have individuals who are inspired by the ideology but aren't directly connected to any specific group," said John Cohen, a former Homeland Security Department 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. In the United States, only about 150 Americans have tried or succeeded in doing so, and authorities say it's difficult to keep tabs on all of them.

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 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 into 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, U.S. free speech and civil liberties guarantees set a high bar for surveillance of potential extremists. The FBI had looked into the activities of the brothers later accused in the Boston Marathon bombing but closed the case after finding no basis to proceed.

Cohen and other former law enforcement officials say U.S. authorities need to burrow deep into ethnic communities and detect radicalization before it metastasizes. It's a combination of surveillance, source development and grassroots outreach, they say.

"How we prevent this is to develop good network of human sources who are going to assist us voluntarily," said David Gomez, a former senior FBI official.

In the Boston, Fort Hood, Times Square cases and others, Cohen said, the investigations showed that people close to the perpetrators noticed disturbing behavioral chances but failed to report them. Often, relatives are afraid their loved one will be arrested, he said.

"We need to change the way we deal with these issues and move beyond the 'report and arrest' model to more of a construct of intervention and violence prevention," he said.