NEW YORK -- The attempted attack on a provocative cartoon contest in Texas appears to reflect a scenario that has long troubled national security officials: A do-it-yourself terror plot, inspired by the the Islamic State of Iraq and Syria (ISIS) and facilitated through the ease of social media.
Trying to gauge which individuals in the United States pose such threats - and how vigorously they should be monitored - is a daunting challenge for counterterrorism agencies. Some experts caution that a limited number of small-scale attacks are likely to continue.
Michael McCaul, chairman of the House Homeland Security Committee, said federal authorities are aware of "thousands" of potential extremists living in the U.S., only a small portion of whom are under active surveillance.
Concerns have been intensifying since the rise of Islamic State group and were heightened this week after two gunmen were shot dead while trying to attack the event in Garland, Texas, that featured cartoon images of the Prophet Muhammad. One of the men, Elton Simpson, was arrested in 2010 after being the focus of a four-year terror investigation; investigators are trying to determine the extent of any terror-related ties involving him or his accomplice, Nadir Soofi.
At the White House, Press Secretary Josh Earnest said Wednesday that intelligence officials would be investigating Islamic State's claim of responsibility for the incident.
"This is consistent with what has previously been described as a lone wolf attack," Earnest said. "Essentially you have two individuals that don't appear to be part of a broader conspiracy, and identifying those individuals and keeping tabs on them is difficult work."
Terrorism experts say the spread of social media - and savvy use of it by extremist groups - has facilitated a new wave of relatively small-scale plots that are potentially easy to carry out and harder for law enforcement to anticipate.
While plots orchestrated by al-Qaida have historically involved grand plans designed to yield mass carnage - airline bombings, for instance, or attacks on transportation systems - the Islamic State group has endorsed less ambitious efforts that its leaders say can have the same terrorizing effect on Western society.
"If you can get your hands on a weapon, how is the state security apparatus supposed to find you?" said Will McCants, a fellow for the Center for Middle East Policy at the Brookings Institution in Washington. "It's attractive because it gets just as much attention as a small- to mid-size bomb."
A public forum like Twitter, with its millions of followers, means those who might otherwise have had limited exposure to terrorist ideologies now have ample access to what FBI Director James Comey has described as ISIS' "siren song." Social media provides a venue for agitators to exhort each other to action, recruit followers for violence and scout locations for potential attacks.
"The speed with which someone can find an active jihadist and connect with them over Twitter, let's say, and start direct messaging with them - that speed happens much faster now," McCants said.
CBS News security analyst Mike Morell, former Deputy Director of the CIA, said ISIS' social media ability is the most sophisticated security agencies have seen.
This phenomenon poses a challenge for investigators as they sift through countless online communications.
"Where is the threshold of saying this is more than just an avid consumer of propaganda?" asked William Braniff, executive director of a terrorism research center at the University of Maryland and a former instructor at the U.S. Military Academy's Combating Terrorism Center.
"It's exceptionally difficult to estimate of the number of people who've considered becoming foreign fighters," he said. "Often you're not dealing with specific behaviors, but with expressions of belief, which are constitutionally protected."
U.S. officials say that more than 3,400 people from Western countries - including nearly 180 from the U.S. - have gone to Syria or Iraq, or attempted to do so, to fight on behalf of ISIS or other extremists groups.
Although there is concern that fighters returning to the U.S. might pose a terrorism threat, some national security experts say a more immediate danger is posed by individuals in America who are inspired by these extremist groups yet have no direct ties to them.
Such individuals "can be motivated to action, with little to no warning," National Counterterrorism Center director Nicholas Rasmussen told the House Committee on Homeland Security in February. "Many of these so-called homegrown violent extremists are lone actors, who can potentially operate undetected and plan and execute a simple attack."
He predicted that the threat posed by these individuals will remain stable, "resulting in fewer than 10 uncoordinated and unsophisticated plots annually from a pool of up to a few hundred individuals."
Meanwhile, Pamela Geller, organizer of the controversial "Draw Muhammad" contest in Texas, faces a new online threat as a suspected ISIS member called for her "slaughter."
Geller met with investigators from the New York Police Department's counterterrorism and intelligence bureaus Wednesday. NYPD spokesman Stephen Davis said they will do a "comprehensive threat assessment" to determine whether Geller will need security, but did not comment on details of the threat or security measures.
Geller is a leader of the American Freedom Defense Initiative, a controversial organization which has run ads in cities across the country criticizing Islam. Geller said she will not back down after the failed Texas attack on Sunday left two gunmen dead. Authorities are still trying to determine whether the attack was directed or inspired by ISIS.
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