An attempted attack by two suspected Islamist gunmen in Texas underscores the persistent threat posed by "homegrown" extremists and the difficulties US authorities face in trying to thwart them, experts said Monday.
Investigators were still trying to piece together the details of Sunday's incident in a Dallas suburb in which a traffic policeman shot dead two heavily armed gunmen who had tried to storm an event organized by an anti-Muslim group.
Amid reports one of the attackers had been suspected of planning to travel to Somalia to fight with jihadists, analysts said it resembled other cases of homegrown militants taking action without orders from abroad.
Senior US officials have repeatedly warned of a growing danger from homegrown Islamist extremists, particularly "lone wolves" acting entirely on their own.
"It's a challenge for us given how hard it is to stop these people because they're on the Internet, in their homes," FBI Director James Comey said in an interview last month.
Comey said there had been "good progress" and a number of arrests, but he added: "I worry about the people who may be in their basements radicalizing that I can't see."
There have been at least 63 homegrown jihadist plots or attacks on US soil between 2001 and 2013, according to the Congressional Research Service, and the number markedly increased starting in 2009.
The Islamic State group, Al-Qaeda and Yemen-based Al-Qaeda in the Arabian Peninsula, as well as al-Shabab in Somalia have all issued appeals to supporters "to conduct lone-wolf attacks against the United States and other Western countries," National Intelligence Director James Clapper told lawmakers in February.
Homegrown extremists usually lack the training, the bomb-making expertise or the financial backing to pull off a large-scale attack. But they have the advantage of surprise, as there is no communication with known operatives overseas that can be wiretapped.
Watch: Texas police shoot dead two gunmen at Prophet Muhammad cartoon contest
Muslims in America
The homegrown threat may have expanded but it still remains on a smaller scale compared to European countries, said Max Abrahms of Northeastern University.
"One of the reasons that's the case, perhaps the biggest reason, is that the American Muslim community is relatively happy," Abrahms, an assistant professor of political science, told AFP.
American Muslims tend to be more assimilated and financially better-off than their counterparts across the Atlantic.
But he said events like the one organized on Sunday, involving a contest to draw cartoon caricatures of the Prophet Mohammed, threaten to alienate some Muslims and provoke a violent reaction.
"It's precisely the kind of behavior that risks ostracizing the American Muslim community and puts our counter-terrorism efforts at risk," Abrahms said.
"When the Muslim community feels mistreated, it's more likely to generate extremists."
To tackle the homegrown problem, US authorities tend to employ two methods: sting operations involving undercover FBI agents or busting a suspect on non-terrorist related charges, such immigration or tax violations.
The sting campaigns have resulted in numerous arrests and convictions but have also provoked criticism. Civil liberties groups have questioned whether the FBI has engaged in "entrapment" of people who had no concrete plans or means to carry out an attack.
In a 2010 case, four men were found guilty of planning to shoot down military aircraft at Stewart Airport in Newburgh, New York. An informant posing as an extremist gave them inert C-4 explosives and an inactive Stinger missile.
Working to build trust with Muslim communities as well as recruit informers inside the communities, American authorities have tried to ferret out extremists before they strike. But police have come under criticism for some tactics that rights groups say single out Muslim Americans for spying and suspicion.
Farooque Ahmed, a naturalized US citizen from Pakistan, was sentenced to 23 years in prison in 2011 after he told undercover officers he planned to bomb subway stations in Washington DC. The tip that led authorities to Ahmed came from a source in the local Muslim community.
The FBI chief, Comey, also has called on technology companies not to encrypt their customers' data, arguing it could undermine efforts to monitor social media for potential terror plots -- a controversial proposal opposed by privacy advocates.
Although media attention has focused heavily on the homegrown "jihadists," a study by the New America Foundation concluded more Americans have been killed by terror attacks by right-leading ideologues, including white supremacists, anti-abortion and anti-government militants, than by those inspired by Islamist propaganda.
Since 9/11, right-wing extremists killed 34 people for political reasons. Islamist extremists killed 21 people since the 2001 attacks, the study said.