WASHINGTON » As Major League Baseball’s top players took the field at the All-Star Game in Minneapolis last July, a covert radar system scanned the sky above the 40,000-seat stadium for what security experts said was an emerging threat to public safety: drones.

Using finely tuned detection programs brought in by the Department of Homeland Security, "Operation Foul Ball," as it was known, identified several small, commercial drones flying in the area. Some were similar to the quadcopter that crashed on the White House lawn on Monday.

But the drone detection system — which was considered one of the most advanced in the country and cost several hundred thousand dollars to operate for just that night — had no way of actually stopping drones from flying into the stadium. There was even confusion about whether one of the drones belonged to ESPN.

Confronted with the system’s cost and limitations, baseball officials decided not to use it for the postseason. But, officials said, when a drone flew over at least one playoff game, there was nothing they could do.

The National Football League will not say what type system, if any, it will have in place at the Super Bowl in Phoenix on Sunday, though the Federal Aviation Administration issued a warning this week that anyone flying drones over an NFL game could be "intercepted, detained and interviewed."

While drones have not been used in a terrorist attack on American soil, thwarting them is increasingly becoming a challenge for law enforcement and security officials who are charged with protecting large-scale events like the Super Bowl and high-profile public buildings like the White House. The officials have warned that the low-flying devices could be modified to carry explosives, chemicals, biological agents, guns or cameras.

This month, an analyst for the National Counterterrorism Center told a gathering of military, law enforcement and public utility officials in Arlington, Va., that extremist interest in "unmanned aircraft systems" had spiked recently. In the past 16 months, he told the group, the working group for unmanned aircraft systems threats had grown from four members to 65, according to a participant at the gathering.

"Efforts by terrorists to use drone technology are obviously a concern for NCTC and the intelligence community," said a spokesman for the counterterrorism center, who requested anonymity to discuss security matters. "Our focus remains on identifying these threats and supporting those agencies who are responsible for countering them."

Officials at the White House said the Secret Service’s air security branch had been working for years to find ways to deter low-flying drones that are too small and fly too low to be tracked by conventional radar, which was designed to pick up missiles and planes.

But Monday morning’s incident suggested that the systems in place around the White House were unable to prevent even a toy drone from flying over the wrought-iron fence.

More details of the incident emerged Thursday. The federal worker who has admitted he was operating the drone was in his apartment, a friend said, flying the drone out the window at 3 a.m., when he lost contact with the device and was helpless as it flew away.

"The whole thing just spiraled out of control when he lost contact," said the friend, who asked not to be identified because of the legal issues involved.

The device was later noticed by a Secret Service officer, but the officer had no way of stopping it before it crashed into a tree on the South Lawn.

Law enforcement officials said the individual flying the drone, whom they have not named, did not pose a threat. And they said that most commercially available drones were unable to carry enough explosives to significantly damage the White House.

The vast majority of drones are purchased for nonthreatening, personal use, and businesses are expressing interest in using drones to speed deliveries, produce high-quality videos or improve management of crops.

But officials are not taking the threat lightly. A White House spokesman said Wednesday that the Secret Service was "constantly reviewing emerging technologies." Law enforcement officials said that the government did not yet have a "silver bullet" that could stop a drone in its tracks.

At the gathering in Arlington this month, intelligence and law enforcement officials showed a widely-watched YouTube video of a masked man demonstrating the effectiveness of a hobbyist drone equipped with a paintball gun and a remote trigger. The video showed the drone accurately firing on cutouts of human beings.

A spokesman for DJI, which manufactured the drone that crashed at the White House, said Wednesday that his company takes security issues seriously and had issued updates to its software that would use GPS chips in the drones to block their use in much of Washington. The company already issued similar blocks near 2,000 airports worldwide.

"The platform will recognize when it’s a certain distance from those no-fly zones and stop itself," said Michael Perry, a spokesman for the company.

Perry said that his company had been in contact with law enforcement about ways to develop protections against the misuse of its drones. But he said the company would never be able to make sure that people only used the drones responsibly, for conservation efforts, firefighting, search and rescue, sports or recreation.

"Ultimately, it depends on how people choose to use the technology," Perry said. "We can create 100 barriers to someone using this technology improperly, and someone will find the 101st way to use it improperly."

For law enforcement officials, the question is whether they can develop systems that effectively detect — and eventually stop — drones approaching potential targets. Several companies, including DroneShield, advertise commercial systems that can detect and warn about drones.

"Our solution would have been able to warn people before the drone got over the property," Brian Hearing, a founder of DroneShield, said this week after the White House incident.

The problem with the system that was used at the All-Star game last summer was that even after a drone was spotted on radar, security officials had to find its operator on the ground before the drone was maneuvered over the stadium.

Jeffrey B. Miller, the chief of security for the NFL, said that the league was increasingly finding drones at stadiums and that it banned them from team properties before the season. In the past year, though, 12 drones have landed around stadiums on game days, he said.

Miller said the league had adopted protocols for what to do if a drone landed on the field during a game. He said that play would be stopped and law enforcement officers who specialize in explosives would be called to determine whether it posed a threat.

"It’s always been hobbyists or enthusiasts," he said, "never anyone looking to do harm."

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