Enemy Inside: ISIS the 'Greatest Threat since 9/11'

The United States is investigating reports of a second American killed in Syria while fighting for the terrorist group Islamic State, also known as ISIS.

Authorities have already confirmed American Douglas McAuthur McCain was killed while fighting with the jihadist army.

Meanwhile, U.S. leaders say they're using every tool to convince American citizens not to travel overseas to become jihadists.

More than 100 Americans are believed to be fighting for ISIS and other Islamist groups in Syria. Their increasing strength is being compared to al Qaeda leading up to the 9/11 attacks.

"They may have again a place to organize a major attack on the United States, and that's very serious," former New Jersey Gov. Thomas Kean, co-chair of the 9/11 Commission, said.

"ISIS presents the greatest threat that we've seen since 9/11," Rep. Michael McCaul, R-Texas, who chairs the House Homeland Security Committee, said.

Could ISIS terrorists be as big a threat to America as al Qaeda? CBN News's Erick Stackelbeck addresses that question and more on The 700 Club, Aug. 28.

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Overall, thousands of Westerners have joined the fighting in Iraq and Syria on the side of ISIS.

Rep. McCaul said keeping the fighting overseas through military pressure and political reform is crucial.

"They are true jihadists, but they have greater access to Turkey, Western Europe," he said. "They have legal travel documents. Some of them have already traveled to the United States and back and want to suicide bomb us."

"Intelligence officials and myself are very concerned about an imminent attack that could happen in the United States if we don't stop them overseas," he added.

Earlier this summer, the Syrian al Qaeda group said Mohammad Abu-Salha, an American from Florida, carried out a suicide bomb attack for them. He released the following threatening video message against the United States.

"You think you are safe; you are not," he warned.

Meanwhile, a desperate mother is appealing to ISIS jihadists to free her kidnapped son, American journalist Steven Sotloff.

"Please release my child," Shirley Sotloff said in a video released Wednesday. "He is an honorable man and has always tried to help the weak."

The White House said the United States will not pay a ransom for hostages, but officials say they will exhaust every other option.

President Barack Obama has authorized surveillance flights over Syria. U.S. officials say they need more information before launching airstrikes against ISIS headquarters there.

The terrorist group has been gaining ground in Iraq and Syria, and now with the aid of American jihadists, the question is not if ISIS is a threat to the United States, but how great a threat it is to the American homefront.