The Direct of National Intelligence, James Clapper, said late Thursday, "The unauthorized disclosure of a top secret U.S. court document threatens potentially long-lasting and irreversible harm to our ability to identify and respond to the many threats facing our nation."
Concerning one such threat, three years ago U.S. officials said the government used email intercepts to track and eventually catch Najibullah Zazi, a Denver man who pleaded guilty to planning to bomb the New York City subway in 2009.
"If that intercept had never happened, Zazi [and his U.S. al Qaeda cell] would almost certainly have conducted three suicide attacks in New York City," said Seth Jones, terrorism expert and author of "Hunting in the Shadows: Pursuit of Al Qa'ida Since 9/11".
Clapper said the government is only authorized to sift through the mountain of phone and internet records it collects on Americans "when there is a reasonable suspicion, based on specific facts, that the particular basis for the query is associated with a foreign terrorist organization."
An FBI spokesperson said he expects an investigation is underway to identify the leaker or leakers.