Kelly explained he was referring to crop dusters that could be commandeered and used to disperse anthrax or some other toxic material. In the months after the Sept. 11 terror attacks, authorities uncovered information from would-be hijacker Zacarias Moussaoui about small aircraft being used this way. It led to the brief grounding of crop dusters nationwide.
In response, Kelly said, New York Police Department officers spoke to local businesses and went to insecticide conferences around the country to learn how the planes were used, bought and manufactured. They also sent officers to Fort Dix in New Jersey to train on shooting from aircraft, and equipped helicopters so they could be armed with heavy machine guns if necessary.
"We didn't want to be totally defenseless in a situation like that, so we have trained our flight crews to use a .50-caliber weapon," Kelly told reporters on Wednesday.
The decision to fire would be only in an extreme circumstance, and would be made in consultation with Mayor Michael Bloomberg, and with the Federal Aviation Administration and other federal authorities, he said.
"We have a hotline to the FAA and we have direct contact to the towers at LaGuardia and Kennedy airport," Kelly said.
He stressed that the weapons are not meant to shoot down a commercial airliner.
"I don't see that as being a real possibility," he said.
The arming of helicopters is only one example of how the NYPD has pursued an ambitious program to fortify defenses against another terror attack against the city.
After the commando-style assault on Mumbai in 2008, the department trained scores of additional officers on the use of high-powered rifles loaded with armor-piercing bullets. The officers have been put through drills at a facility that simulates a typical New York City block.
The use of motorboats by terrorists in Mumbai also prompted New York police to order a high-speed combat vessel that can carry up to 30 officers armed with heavy weapons. Officials say the $5 million boat gives the department the ability to respond with rapid force if the Statue of Liberty, Ellis Island or other sites are faced with a threat like this year's Norway massacre.
Although the Daily News of New York had reported in 2005 that a revamping of NYPD's helicopter fleet included giving choppers heavy weapons capabilities, police officials had kept the idea under wraps until Kelly's remarks in a "60 Minutes" segment that aired Sunday on CBS.
Asked about potential threats from aircraft, Kelly said: "Well, it's something that's on our radar screen. I mean in an extreme situation, you would have some means to take down a plane." But he declined to go into details, raising questions about whether the department had the authority and expertise to shoot down a plane.
Bloomberg said earlier this week that since Sept. 11, the NYPD has developed counterterror defenses the public may never know about.
"The New York City Police Department has lots of capabilities you don't know about and you won't know about them," he said.
The department has seven helicopters in its fleet, which include Bell- and Augusta-made choppers. Not all are equipped to handle the weapons.