NSA chief: Surveillance has helped stop more than 50 terror attacks

Alexander noted last week in Senate testimony that the surveillance programs helped stop dozens of terror plots.

He briefly mentioned planning to bomb the New York subway system, but fuller details about that and revelations about others emerged on Tuesday in the House.

In all, officials said the controversial surveillance aimed at communications overseas helped to disrupt more than 50 plots globally that were in various stages of planning.

Details of virtually all remain secret, but national security officials said they were working on declassifying more information and could have a report to Congress as early as this week.