The attacks in Paris by the Islamic State group have stoked fears of a similar attack on U.S. soil, but Defense Secretary Ash Carter said Monday it is unlikely the militants have such capability.

"They say they have the inspiration to come here," Carter said during a talk at the Wall Street Journal CEO Council. "Their capability (in the U.S.) is not what it is in Europe. There isn't as much ease geographically in the movement of people."

The attacks in the French capital on Friday, for which the Islamic State group claimed responsibility, took the lives of at least 129 people at restaurants, a soccer stadium and a concert hall. The massacres renewed questions about how to improve intelligence gathering and anticipate future attempts by terrorist groups such as the Islamic State, also called ISIS and ISIL.

Carter acknowledged the controversy surrounding the National Security Agency's gathering of metadata from the phone records of millions of Americans in addition to its foreign intelligence operations, but he said the Islamic State's ability to avoid detection by intelligence agencies is a problem for America's national security and requires extraordinary measures..

"We're trying to protect our country and our people, and we need to be reasonable about that," he said. "We need to find a way that is consistent with a free and open Internet, but which also allows us as public officials to protect our people."

Florida Sen. Marco Rubio, one of the Republicans vying for his party's presidential nomination, was pessimistic about the state of U.S. intelligence operations when he spoke at the CEO Council earlier Monday.

"We are vulnerable," Rubio said. "What happened in Paris can happen in an American city at any moment at any time. We have weakened the U.S. intelligence-gathering capability."

Rubio emphasized the need for access or back doors to user data from companies such as Apple.

"The United States government has neither the competence, the money nor the time to spy on every American. That's not happening," Rubio said. "But we need to have access to this information in order to save lives."

Colin Clarke, a political scientist with the Rand Corp., echoed Carter's belief that the geographic distance between the U.S. and Europe and the Middle East is an advantage, but it is not an impossible impediment for a calculating opponent like the Islamic State.

He said obvious targets are the Super Bowl and the National Mall of America, but noted that the Paris attacks on ordinary eateries and entertainment venues change expectations.

"What about those everyday targets, how do you figure out what's valuable and what's not?" Clarke said in a phone interview. "Frankly, we can't protect everything, and we've got to go based on intel."

Carter said that in addition to aggressive intelligence operations, digging deeper into social platforms and encryption services are key to defeating social media-savvy terrorist groups like ISIS.

"In the terrorist space, this has turned out to be a very ugly capability for people like this to have," Carter said. "An organized, civilized society has to figure out how to protect itself from this kind of stuff."

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