Homeland Warns of ISIS Retaliation in US by Sympathizers US fears that a radicalized sympathizer could attack homeland.

 -- Federal authorities today urged law enforcement across the country to be alert for possible attacks inside the United States in retaliation for U.S. airstrikes against the Islamic group ISIS, the brutal terrorist group that beheaded American journalist James Foley and has seized vast swaths of territory in Iraq and Syria.

In a joint bulletin issued to local, state and federal law enforcement, the Department of Homeland Security and FBI said that while they are “unaware of any specific, credible threats against the Homeland” and find most threats to the U.S. homeland by supporters of ISIS “not credible,” they cannot rule out attacks in the United States from sympathizers radicalized by the group’s online propaganda.

“[B]ecause of the individualized nature of the radicalization process – it is difficult to predict triggers that will contribute to [homegrown violent extremists] attempting acts of violence,” the bulletin states. Moreover, such lone offenders “present law enforcement with limited opportunities to detect and disrupt plots, which frequently involve simple plotting against targets of opportunity,” according to the bulletin.

The group is known by the acronyms ISIS and ISIL and recently changed its name to the Islamic State, claiming it has formed an Islamic caliphate in the areas of Syria and Iraq that it controls.

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The beheading of Foley was in retaliation for U.S. air attacks on ISIS fighters, according to a statement from the group. ISIS is holding at least one other American journalist, Steven Soltoff, and is threatening to kill him.

Such Homeland Security bulletins outlining potential concerns from U.S. officials and urging increased vigilance are common – essentially acting as a generic “FYI” to local, state and federal law enforcement who don’t have the same access to classified information that the nation’s top counterterrorism officials see every day.

The bulletin issued today noted that ISIS’ English-language messaging online “has primarily been focused on calls” for Muslims to join the group in Iraq and Syria. The group and its supporters, meanwhile, “have employed – and will almost certainly continue – Twitter ‘hashtag’ campaigns that have... been able to quickly reach a global audience of potential violent extremists,” the bulletin said.

In the joint bulletin, DHS and FBI mentioned the propaganda video released this week showing Foley’s beheading, and it cited a photo posted online showing the ISIS (ISIL) flag in front of the White House.

“ISIL members and supporters will almost certainly continue to use social media platforms to disseminate their English-language violent extremist messages,” the bulletin concluded. “Although we remind first responders that content not explicitly calling for violence may be constitutionally protected, we encourage awareness of media advocating violent extremist acts in particular locations or naming particular targets, to increase our ability to identify and disrupt potential Homeland threats. We urge state and local authorities to promptly report suspicious activities related to homeland plotting and individuals interested in traveling to overseas conflict zones, such as Syria or Iraq, to fight with foreign terrorist organizations.”

In addition, the bulletin said that “civilian deaths reportedly associated with these U.S. military air strikes will almost certainly be used as further examples of a perceived Western war against Islam in English-language violent extremist messaging that could contribute to [homegrown] radicalization to violence.”

ISIS has issued threats against the U.S. homeland before. In a recent video series, Vice Media embedded with ISIS militants and recorded an ISIS militant saying, "God willing, we will raise the flag of Allah in the White House."

ABC News’ Chris Good contributed to this report