The skies over San Diego are crowded most every day with commercial airliners, military planes, private aircraft and helicopters crisscrossing over scores of separate flight paths.

Among the airborne traffic: aircraft belonging to the FBI and the Department of Homeland Security , outfitted with sophisticated video cameras and — in some instances — cell- phone tracking devices .

That’s the conclusion of a story published this week by the news website BuzzFeed. The website analyzed aircraft location data and concluded some 200 federal aircraft take to the skies daily and slowly circle at low altitudes over many American cities, including San Diego.

Red lines: FBI aircraft | Blue lines: Homeland Security

The data is a snapshot, from mid-August through the end of the year, that depicts flights made by the FBI and Homeland Security. The lines become thicker and darker the more frequent that the flight paths were used.

The picture over San Diego County shows a spider web of intersecting lines, concentrated largely on the part of the county west of the mountains.

Two items stand out: a thick circle of FBI flights over downtown San Diego, and a larger Homeland Security circle that passes over Hillcrest, Montgomery Field and the University of California San Diego.

Why precisely the two agencies were making repeated trips over those areas during that time isn’t known. Homeland Security wouldn’t comment on the reason for flights over specific cities, but said in a statement to BuzzFeed that the flights are used as part of investigations into human trafficking, drug smuggling and violent crimes.

The FBI said that it uses flights to support criminal investigations. When asked about the San Diego patterns, the local FBI office provided a statement first released by the agency in June 2015 when news of the flights first surfaced.

The statement said that it should not be a surprise to know it uses the planes in criminal probes, and refuted the notion it was conducting mass surveillance.

“These aircraft are not equipped, designed, or used for bulk collection activities or mass surveillance, and are not routinely equipped with cell site simulators,” the statement said. “With a court order, or under exigent circumstances such as a hostage situation, the aircraft can be fitted to include a cell site simulator. However, this is rarely utilized, and only with senior level approval. “The FBI does not monitor lawfully protected First Amendment activity.”

So what gives? It’s hard to know — and that is precisely the issue, said Nicole Ozer, the Technology & Civil Liberties Policy Director for the ACLU of California, which has long been critical of government surveillance programs cloaked in secrecy.

“We don’t know what they’re doing, and to me that highlights the fundamental issues here,” she said. “The federal government is spending billions of dollars on surveillance technology, and handing out millions in grants to local police and sheriff's departments. And basic transparency, accountability and oversight are non-existent.”

In San Diego, the border area has long been patrolled from the air by Customs and Border Protection — the agency that includes the Border Patrol. The BuzzFeed map showed FBI flights concentrated mostly in central San Diego and East County, just east of El Cajon, and another pocket of activity over Oceanside, south of Camp Pendleton.