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The American military has stepped up security measures at installations around the country in response to growing concerns about "homegrown violent extremists," a U.S. defense official said.

The commander of the U.S. Northern Command sent an advisory Thursday night directing all commanders in the United States to tighten up protection, the official said.

The increased vigilance is not specifically tied to a threat from ISIS, but it is based on recent intelligence FBI Director James Comey mentioned earlier this week regarding the collection of intelligence on people in America who sympathize with ISIS and other Islamic terror groups.

A plot by two Phoenix men to attack an anti-Prophet Muhammed event in Garland, Texas was foiled Sunday when a police officer shot them dead at the scene. Comey said this week that the FBI had opened an investigation of one the gunmen, Elton Simpson, after he made statements on social media indicating an interest in jihad. Hours before the attack, the FBI alerted local police that Simpson was "interested in the event," but the agency had no reason to believe he was going to attack it, Comey said.

The FBI and Department of Homeland Security had also warned law enforcement agencies that supporters of ISIS and other terror groups had posted links to the Garland event on Twitter.

Comey said the case reflected the change in nature of homegrown terror threats, in which potential attackers connect with recruiters through social media.

The FBI has hundreds of investigations of potential home grown extremists under way, with cases open in every state, Comey said. "I know there are other Elton Simpsons out there," he said.

The U.S. military "shares the same concern about the potential threat posed by homegrown violent extremists," the defense official said Friday.

The official added that the military's "Fore Protection Condition" was heightened not because of a specific threat, and is a "prudent measure to remind installation commanders" of the importance of "increased vigilance and safeguarding of all (Department of Defense) personnel, installations and facilities."

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— Jim Miklaszewski, Courtney Kube and Jon Schuppe