Intelligence officials in the United States have laid out a forceful defence of the Obama administration's sweeping phone and internet surveillance programmes.
A panel of intelligence chiefs appeared before the House Intelligence Committee on Tuesday.
National Security Agency director Keith Alexander said the public disclosure of those surveillance programmes by former NSA contractor Edward Snowden had caused irreversible damage to national security.
He said the agency's data gathering programmes had prevented potential terrorist attacks more than 50 times since September 11, 2001.
General Alexander said the agencies would hand over the documents about these cases for the committee's review.
He also said he would discuss two thwarted plots during his testimony.