Fire service spokesman Sven Gerling said his initial information was that two helicopters had crashed, but he had no further details on the accident.
Federal Police spokesman Frank Brochert confirmed there "was an incident during an exercise" and that emergency crews were on the scene. He had no details about the accident.
More top news
Mack entered her plea in federal court in Brooklyn. The judge refused a request by her lawyers to release her without bail.
Officers investigating a reported break-in on Friday found 22-year-old Roger Alvarado asleep in the pop star's home in Tribeca.
Billy Jeeves is said to have fled the scene in Hither Green, south-east London, after Henry Vincent was fatally stabbed by the pensioner.