WASHINGTON (Reuters) – The National Transportation Safety Board said on Thursday it is investigating the Sept. 21 collision of a civilian drone and a U.S. Army UH-60 helicopter east of Staten Island, New York City, as concerns mount over the rising number of unmanned aircraft in U.S. airspace.

The safety board said the Army helicopter sustained damage to its main rotor blade, window frame and transmission deck. The Army is conducting a mishap investigation, the agency said.

Earlier this year, the U.S. Federal Aviation Administration banned drone flights over 133 U.S. military facilities, citing security concerns.

In August, the Pentagon said it had given U.S. military bases the green light to shoot down private and commercial drones that could endanger aviation safety or pose other threats.

The NTSB said it had recovered a motor and arm from the drone – identified as a DJI Phantom 4 – from the helicopter. The NTSB was able to identify and interview the drone operator and has reviewed flight data logs for the flight on which the incident occurred.

Last week, the FAA said it was banning drone flights over 10 U.S. landmarks, including the Statue of Liberty in New York and Mount Rushmore National Memorial in South Dakota, at the request of national security and law enforcement agencies.

The FAA and U.S. Department of the Interior said they would ban drone flights up to 400 feet (122 m) within the boundaries of sites including the USS Constitution in Boston, the Gateway Arch in St. Louis and the Independence National Historical Park in Philadelphia. The list also includes Folsom Dam and Shasta Dam in California, Glen Canyon Dam in Arizona, Hoover Dam in Nevada and Grand Coulee Dam in Washington state. The restrictions are effective Oct. 5.

The number of unmanned aircraft in U.S. skies has soared in recent years and continues to increase rapidly, along with concern among U.S. and private-sector officials that dangerous or even hostile drones could get too close to places like military bases, airports and sports stadiums.