The terror group known as ISIS took credit for the recent failed attack at a Muhammad cartoon event in Texas. Now it may have published a warning online that sleeper cells have been established in 15 states.

According to a report on Fox News, the warning, which was signed by Abu Ibrahim Al Ameriki but could not be independently verified, claimed that ISIS had placed terror cells in five states. The name behind the threat matches an American known to have joined a terror group in Pakistan several years ago and has previously appeared in propaganda videos.

"Out of the 71 trained soldiers 23 have signed up for missions like Sunday, we are increasing in number," the warning stated. "Of the 15 states, 5 we will name... Virginia, Maryland, Illinois, California, and Michigan."

Despite the fact that the attack in Texas was unsuccessful, Fox News reported that ISIS thought otherwise.

"The disbelievers who shot our brothers think that you killed someone untrained, nay, they gave you their bodies in plain view because we were watching," the post stated.

Fox News reported that the post singled out Pamela Geller, the controversial anti-Islamist who helped organize Sunday's cartoon event in Texas.

"The attack by the Islamic State in America is only the beginning of our efforts to establish a wiliyah [province] in the heart of our enemy," read the warning. "Our aim was the khanzeer Pamela Geller and to show her that we don't care what land she hides in or what sky shields her; we will send all our Lions to achieve her slaughter."

According to Fox News, U.S. officials are assessing the threat, although an intelligence source believed that the warning may not have originated from ISIS leadership. The unnamed source indicated that "an opportunist such as a low-level militant" may have made the threat.

"We are focused keenly on who would be looking to travel to join this band of murderers who will have come back from Iraq and Syria and to the United States," FBI Director James Comey said. "We have opened cases all over the place focused on this threat, so it is not ... a Washington thing - it is something we focus on throughout the FBI."

According to Peter Bergen and David Sterman of CNN, public records and news accounted indicated that there are 62 individuals in the U.S. who have attempted to join militant groups in Syria such as ISIS; some have succeeded in joining or recruited others into the group. They noted the diversity of the people who joined the terror group, although many of them tended to be young.

"There is no single ethnic profile for these militants: They are white, African-American, Somali-American, Vietnamese-American, Bosnian-American and Arab-American, among other ethnicities and nationalities," Bergen and Sterman wrote. "An unprecedented number of American women are involved in the Syrian jihad compared to other such jihads in the past."

However, CNN pointed out that American militants who joined ISIS and other terror groups tended to be active in "online jihadist circles."

"Militants in the United States today become radicalized after reading and interacting with propaganda online and have little or no physical interaction with other extremists," Bergen and Sterman wrote.

CNN then focused on the reasons why some Americans would abandon their lives in the U.S. and join the ranks of ISIS.

"ISIS is creating what its recruits believe to be a perfect Islamic state, trying to restore the Caliphate that ceased to exist after the end of World War I and the collapse of the Ottoman Empire," Bergen and Sterman wrote. "ISIS is also even presenting itself as the vanguard of Muslim warriors who will usher in the End of Times and the final, inevitable battle between the West and Islam."

However, CNN reported that those Americans who threw their lot in with ISIS generally faced either certain death or getting caught by U.S. law enforcement on their way to Syria.

"Stopping Americans from a quite likely death after they are lured to Syria by predatory online ISIS recruiters may be a significant justification for focusing resources on this issue, in addition to the more obvious goal of preventing an attack in the United States by a returning fighter from Syria," Bergen and Sterman wrote.