The Federal Bureau of Investigation and Department of Homeland Security are running extensive surveillance missions over US cities, as revealed by a new investigation by Buzzfeed News. The project looked at flight data collected from FlightRadar24, a public tracking service, then zeroed in on flight plans logged by planes registered to the two agencies.

The result is an unusually detailed look at government flights between mid-August and December of 2015, a period that saw more than 3,500 surveillance flights from the two agencies. Those flights would typically circle over a populated area in a holding pattern, occasionally moving as if to follow a specific target. The flights also drop more than 70 percent on the weekends and holidays, suggesting that flights are likely more than just an emergency measure. The flight dataset is available on Github.

The planes are equipped with on-board cameras, but it's unclear what additional surveillance equipment may be onboard. In 2014, The Wall Street Journal reported that the US Marshals service was using Stingray devices on similar flights to locate prisoners within cities, significantly disrupting cellular service in the process.

The data also coincides with the period of the San Bernardino attack on December 2nd. Within 90 minutes of the attack, the data shows two separate planes arriving on the scene of the crime — a Cessna piloted by the FBI and a Pilatus PC-2 belonging to Homeland Security. The planes circled over two other locations in the area, apparently corresponding to the attacker's home and mosque.