SAN DIEGO — U.S. Border Patrol agents based in San Diego have spotted 15 drones flying between Tijuana, Mexico, and Southern California over the past 12 months, according to new data provided to the Washington Examiner.

Not one of the drone operators involved in those incidents was prosecuted for using an unmanned aerial system, or drone, around the international border, which is protected air space.

From Oct. 1, 2016, through Sept. 30, 2017, the same border sector reported two known incidents. In fiscal 2016, five occurred. Before then, it wasn't even tracked.

San Diego Deputy Chief Patrol Agent Roy Villareal said his 2,200 agents have a hard enough time seeing or hearing the small aircraft at night — when they are most often flown — but intercepting them is nearly impossible. In fact, only one smuggler ever has been prosecuted in a drone incident near the border, according to U.S. Customs and Border Protection.

Between being unaware of just how widely cartels are using drones to smuggle contraband over the border and being unable to do anything when they are seen, it's nearly impossible for agents to determine who sent the drone and then legally go after that person.

It's also difficult to know just how great the threat is when agents are not aware of just how widespread drones are being used to spy on them from hundreds of feet above.

“Detection is our greatest weakness right now,” Villareal told the Examiner during a recent tour of the region. “It’s one of a multitude of threats we have, but I think it’s going to grow ... simply because of the inability to intercept [them]."

The person flying a drone can stand a few hundred feet or a couple miles away. He or she can operate the drone with a remote control or input instructions and let the drone operate without a guide. Either way, the owner is able to avoid being captured by U.S. federal law enforcement while smuggling drugs, money, firearms, or other items between countries.

Because drones are small, they typically carry a few ounces to a few pounds of higher-priced drugs.

Villareal said the devices could be used to carry out a massive attack on the public. Someone looking to carrying out a terror attack could plant synthetic fentanyl on the drone and release it over a group of people. Just three tiny grains of the strongest fentanyl being smuggled into the U.S. is enough to put a person in a coma and a small baggie full of the substance would have devastating effects.

“It's the perfect criminal tool,” Villareal said. “A single pound of fentanyl [dropped above a crowd] would devastate a whole stadium.”

Federal law enforcement officers and agents do not have any tools for detecting a drone outside of their eyes and ears. Even if they do spot a drone that appears to be carrying drugs, they can’t shoot it down with their gun. Counterdrone tools exist in the private sector but have not yet been approved for agents to use, leaving them helpless to do anything.

The Department of Homeland Security was given permission in last month's Federal Aviation Administration Reauthorization Act of 2018 to begin looking at what types of threats drones pose, how to detect them, and how to respond. But that process might take a year or so until officials form a plan and fulfill contracts for counterdrone resources.

In 2010 — before drones hit the market — smugglers in Mexico were flying ultralight aircraft packed with marijuana over the border into Arizona’s Tucson and Yuma regions, as well as San Diego.

The aircraft look like go-karts with one large triangle-shaped wing on top. Smugglers would use a small man, often a teenage boy, to fly the load from Tijuana into San Diego.

Villareal said agents had an easier time detecting and catching the pilot because of how loud and visible the machines were when they flew by, even at night. They would drop 50 to 200 pounds of drugs to someone waiting for them on the U.S. side, then take off back to Mexico.

That lasted until 2014 when drones emerged and a few years later became the new method for smuggling illegal or regulated items without using a foot soldier.

Villareal said because the ultralights used lawn mower engines, they were much easier to hear even 100 to 200 feet above ground. Detecting a near-silent drone buzzing by at up to 50 miles per hour is a whole other ball game.