WASHINGTON — As Major League Baseball’s top players took the field at the All-Star Game in Minneapolis in 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 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 those officials had no warning before a drone hovered over at least one playoff game.
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Jeffrey B. Miller, the chief of security for the N.F.L., said that the league was increasingly finding drones at stadiums and that it had banned them from team properties before the season. In the past year, though, 12 drones have landed around stadiums on game days, he said.
Mr. 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.”
A version of this article appears in print on January 30, 2015, on page A1 of the New York edition with the headline: Drones Spotted but Not Halted Raise Concerns. Order Reprints| Today’s Paper|Subscribe
