Mayor Michael R. Bloomberg on Tuesday defended the New York Police Department's monitoring of the Web sites of Muslim student groups at more than a dozen universities across the Northeast, framing the effort as one way to guard against the threat of terrorism.

''The Police Department goes where there are allegations, and they look to see whether those allegations are true,'' Mr. Bloomberg said at an appearance at the Brooklyn Public Library. ''That's what you would expect them to do. That's what you would want them to.''

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