Las Vegas - The head of the National Security Agency is facing hecklers during a speech to a conference of hackers in the US.

Army Gen. Keith Alexander remained unapologetic Wednesday about methods the NSA uses to go after bad guys who aim to kill people.

He told the Black Hat conference that the ability to look for patterns in telephone and email communications has stopped terror attacks in the US and other countries.

Some people laughed at his comments, and someone shouted that he should read the Constitution. Alexander said he had and that the heckler should, too.

He insisted that it wasn't true the spy agency listens to specific phone calls and reads emails.

Sapa-AP

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