The City of Boston had extra security at the Boston Marathon today, with 4,000 uniformed and plainclothes police officers and 100 police dog units lining the 26.2-mile (40.2 km) course—twice as many as last year’s event, when two bombs that exploded near the finish line killed three and wounded 260. Low-flying helicopters are keeping watch from above, while a radiation-seeking chopper flies over the course, trying to pick up spikes in radiation levels that weren’t there during its initial flyby on Thursday.

But what’s missing—at least as far as we know—is a security measure that some experts say will become inevitable in time: drones. ”Drones are going to be the standard,” said Joseph F. King, a criminal justice professor at Manhattan’s John Jay College who has served as chief of the national security section in New York’s Department of Homeland Security office.

Sprawling, citywide events such as marathons present a particularly thorny security problem, King told Quartz. “You can’t just set up roadblocks and search everybody going to the marathon,” he said. “You’re talking about a dense population; it’s as simple as crowd control. If you put in a drone, you can zero in a camera on a particular individual. The operator can directly contact the local sector sergeant and say, ‘move officers here.’”

Boston police commissioner Edward Davis said after last year’s tragedy that “drones are a great idea” for a future marathon, though that idea has reportedly since been turned down. Several other municipalities and local law-enforcement agencies are among the many and varied entities that have applied for licenses to operate drones, however—including the Seattle police, the Miami-Dade police, and the Clackamas County sheriff’s office in Oregon.

There’s much speculation over whether New York Police Department (NYPD), which is charged with protecting the city’s annual marathon in November, will turn to drone security. Shortly after last year’s marathon, tech site Motherboard called it ”an open secret that New York’s finest have been eyeing drones as prospective aerial surveillance tools.” It cited another report indicating that the NYPD’s counterterrorism division has talked to the Federal Aviation Administration about the possibility of using drones. Boosting the speculation, New York’s then mayor, Michael Bloomberg, called drone surveillance “scary” but inevitable on his radio program, opining that “you can’t keep the tides from coming in.”

It remains to be seen whether Bloomberg’s successor, Bill de Blasio, will buy into the idea, King said. Though no one wants to see a repeat of the 2013 Boston Marathon disaster, “There are political repercussions of having a spy in the sky—Big Brother, and all that.”

Neither the NYPD nor the Boston police had responded at publication time to requests for comment.