To an extraordinary extent -- and with good reason -- the city considers itself on permanent alert, with a high level of vigilance that is our best protection.

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The nation's civilian "high" alert -- the second highest level of warning -- was put in place Tuesday.

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Federal officials lowered the nation's health alert level Monday from red, or "high, " to orange, or "elevated, " and said university and secondary students can return to class Thursday while younger students will wait until May 11.

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New York has been on "orange" alert, or the second-highest level -- indicating a high risk of terror attack -- since the color-coded warning system was established after the September 11, 2001 attacks.

CNN: New York subway system on alert