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ISIS' claim of responsibility for the shooting in Garland, Texas, was proof of its effectiveness at capitalizing on so-called lone wolf attacks — even when it has no hand in planning or paying for them, experts said Tuesday.

ISIS provided no evidence to support its claim after the shooting on Sunday at a Prophet Muhammad cartoon contest. But experts told NBC News that accepting responsibility was a cheap way for the militants to achieve their terrorist goals.

"Lone wolves are a low-cost, low-resource way to carry out attacks, where the group can then retroactively decide if it wants to claim responsibility or not," said J.M. Berger, co-author of "ISIS: The State of Terror" and a nonresident fellow at the Brookings Institution.

"If they like it," he said, "they can adopt it after the fact."

It was the first time ISIS claimed credit for an attack inside the United States. The White House was skeptical: A spokesman said Tuesday that it was too early to determine whether the terror group was directly involved.

The two gunmen were shot and killed by a police officer after they hit a security guard in the leg.

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Inspiring lone-wolf attackers has been "the holy grail of terrorist groups for decades" because they can generate destruction and worldwide publicity without spending money or resources, Berger said.

ISIS has had unparalleled success, according to Berger and Daveed Gartenstein-Ross, senior fellow at the Foundation for Defense of Democracies. Ross said that's mostly because of ISIS' "very good social-media game."

Twitter and other sites are particularly effective because ISIS can use its considerable clout to issue widespread messages urging its supporters to attack. Meanwhile, other radically minded users act as a peer-pressure group to encourage possible perpetrators.

"In an act of terror, where you might be killed or spend the rest of your life in jail, you need other people to egg you on," Gartenstein-Ross said. "This allows ISIS to effectively carry out terror attacks at no cost."

Before the Garland shooting, ISIS claimed responsibility for — or at least praised — attacks in Sydney, Tunisia and Libya. The Texas attack appears to be "ISIS-inspired rather than ISIS-directed," Garnstien-Ross said. "These guys didn't receive any training from ISIS."

ISIS typically claims responsibility when attackers have posted messages, usually on Twitter, pledging allegiance to the group or its leader, Abu Bakr al-Baghdadi.

Elton Simpson, one of the suspected gunmen in the Garland shooting, appeared to have communicated with at least one known ISIS member on the social networking site, the experts interviewed by NBC News said.

"The only reason that Islamic State has claimed responsibility is because of the tweet from what we believe to be Simpson’s Twitter account," said Charlie Winter, an NBC News counterterrorism consultant and a researcher at the London-based Quilliam Foundation think tank.

Winter noted that there was no similar claim from ISIS after the Charlie Hebdo attack in January, which was later linked to al Qaeda in the Arabian Peninsula.

"It just takes someone to tweet that they pledge allegiance to al-Baghdadi and then it gets rebranded as an Islamic State attack," Winter said. "It makes the group look much more ubiquitous."

Berger warned that this was no reason to discount the possibility of an overseas attack planned and funded by ISIS' leadership.

"ISIS has a tremendous amount of manpower and resources, and a lot of money," Berger said. "So it’s not safe to assume that ISIS isn't planning global attacks as well."

To put in perspective the relative wealth of resources at ISIS' disposal, Berger said: "They can invest a fraction of their manpower and money and still match the resources that were spent on 9/11."