New York Police Commissioner Ray Kelly took a lot of watchers of 60 Minutes by surprise on Sunday night when he confirmed to Scott Pelley that the New York Police Department had the capability to shoot down an aircraft. He wouldn't go into details, but confirmed that the department had the "equipment and training," and said, "obviously this would be in a very extreme situation." On Monday, Mayor Michael Bloomberg answered the natural follow-up question when he told reporters that the NYPD's air power "certainly does not mean 9/11 wouldn't have occurred." Then the mayor got a little testy, telling the New York Post, "It's not my job to rehash history."
Update: Wired's Noah Shachtman points out that the NYPD's method for shooting down a plane involves moutning a .50 caliber sniper rifle on a helicopter and flying getting off a shot at the engine. It's conceivable, but not likely, and certainly wouldn't work on a jetliner.