Known terrorist groups already have begun to change the way they communicate in the wake of classified leaks detailing U.S. surveillance tactics, U.S. intelligence and counterterrorism officials tell Fox News.
"We are already seeing indications that they are attempting to change their communications behaviors," said one senior U.S. official, speaking to Fox News on the condition of anonymity. "That is a direct result of what we are seeing in the media. That is a fact."
Former NSA contractor Edward Snowden has acknowledged providing sensitive information to the media on U.S. surveillance programs. Facing federal charges in the U.S., Snowden continues to evade capture and is said to be in a Moscow airport.
Counterterrorism officials say that although some of the basic principles of U.S. surveillance were known before the leaks, terrorist groups are now armed with new details that can help them keep their communications private.
"They now know the scope and breadth of our abilities and our collection," one official said.
Terror and extremist groups are likely to become must more cautious with Internet and telephone communications, considering revelations on just how much so-called "meta-data" the NSA has legal justification to collect. Officials told Fox News that terrorists, while routinely careful of their communications, would have been unaware about the sheer scope of the NSA's blanket warrants with Internet giants like Google and Facebook, or phone companies like AT&T and Verizon.
"The real-world implication is these people will stop talking and change how they communicate," said a senior U.S. official. "We are going to have less abilities if they change electronically and until we are able to regain communications we will miss what they are saying. We will miss those dots."
One fear is that terror networks could turn to couriers, similar to the system used by the former Al Qaeda leader Usama bin Laden. Although it was the courier that eventually led U.S. Special Forces to his compound in Abbottabad, Pakistan, it took over 10 years to find him.
At a Pentagon press conference Tuesday, Gen. Ray Odierno, chief of staff of the Army, said he is concerned about the safety of U.S. troops because of this leak.
"It's not just about leaking information," Odierno said. "It's much bigger than that. … It puts American soldiers, sailors, airmen and Marines at risk who are overseas conducting operations."
In an interview on CNN Monday, Secretary of State John Kerry went so far as to say, "people may die as a consequence of what this man did."