A recent survey of drone activity near U.S. military bases revealed that the Defense Department detected nearly 100 unauthorized drones flying near the Pentagon over a two-month period in 2017.

The study followed a ban by the Federal Aviation Administration of unauthorized drone flights over 133 military bases, which was implemented amid concerns that terrorists and spies could employ drones for surveillance, espionage and attacks, Stars and Stripes reported.

Although officials did not provide a list of the military bases involved in the survey, Michael Howard, public affairs director at Joint Base Myer-Henderson Hall, said that drone-detection equipment was deployed last year at two bases near the nation’s capital — Fort Lesley J. McNair from July 19 to Aug. 24 and Joint Base Myer-Henderson Hall from Aug. 24 to Sept. 23.

“We detected 95 instances of drone activity,” Howard said. “We did not attempt to determine the nature of the drone activity, whether it was recreational or something else.”

McNair and Myer-Henderson Hall are located less than two miles from the Pentagon and Ronald Reagan Washington National Airport and inside the 15-mile radius inner ring of a “no drone zone” where flights of remotely piloted aircraft are prohibited “without specific FAA authorization.”

According to Howard, the purpose of the study was to determine if drones were being deployed over military installations and to initiate a dialogue regarding threats that drones might present to the bases.

Pablo Estrada, a spokesman for airspace security company Dedrone, which assisted the Defense Department in conducting the survey, noted concerns regarding U.S. adversaries utilizing drones to monitor troop movements, supply levels or training exercises inside military facilities.

Dedrone’s sensors monitor radio signals from drones and are capable of determining their manufacturer, model and distance, Estrada noted, adding that many of the drones detected near the military installations were small models that consumers can easily obtain.