Many of the operators were probably oblivious to the flight ban or just ignoring it as they flew for fun, he said. But he's not sure.
"Are they bad guys? Well, we don't know," Duggan said. "It's a technology that can be used to attack us at home. Why? Because we are not as prepared as we need to be."
In an acknowledgment of the threat, Congress in November voted to broadly expand the Defense Department's anti-drone powers within the United States. President Trump signed the measure, included in a major defense bill, last month .
Millions of agile and easy-to-fly quadcopters and other drones are sold in the United States each year, with Christmas providing the latest boost. Yet many of the quandaries that come with the devices have not been addressed. Although any tool or technology, from a rifle to a rental truck, can be misused, security experts say drones have introduced broad new dangers and have outpaced efforts to regulate them.
Whether controlled by remote or set to fly autonomously, many drones can carry surveillance cameras, hacking devices or explosives long distances, easily evading ground defenses, experts say. And, they add, the threat from what officials call "unmanned aircraft systems" (UAS) is not theoretical.
An analysis prepared by members of a team at Eglin Air Force Base in Florida specializing in counter-drone operations reported in 2016 that "critical assets within the continental United States have already been 'attacked' by nefarious UAS operators."
Members declined to specify the targets or provide details, given security and other concerns.
"While no deaths have been attributed to these UASs, it is only a matter of time before these systems are directly or indirectly responsible for loss of life or interference with critical infrastructure in the homeland," the analysis said.
In testimony before the House Armed Services Committee in March, a top military leader voiced concern about recent unauthorized drone flights over Navy and Air Force installations.
"These intrusions represent a growing threat to the safety and security of nuclear weapons and personnel," said John Hyten, commander of the U.S. Strategic Command.
In the almost three years since a recreational drone user crashed his two-foot-wide quadcopter on the White House grounds early one January morning — and called to "self-report" the incident to the Secret Service six hours later — federal authorities have been trying to figure out how best to protect the capital.
A few months after that incident, in Tokyo, a protester was arrested for landing a drone carrying a harmless amount of radioactive cesium on the prime minister's office.
It's tough to tease out the potential attackers from the "knuckleheads," and officials say the U.S. government has been hamstrung as it works to upgrade security.
Drones inhabit a curious space in U.S. law, making them particularly difficult to regulate. They have been deemed "aircraft," just like a Boeing 787, so they can't simply be knocked from the sky. Sometimes dubbed "flying laptops," they also are covered by laws against wiretapping and computer hacking. And most drones are categorized as "model aircraft," but Congress has said the Federal Aviation Administration generally can't issue regulations covering those.
That all complicates security efforts and strictly limits what data can be pried from them to track their users or seize their controls. At the same time, opening up laws protecting electronic communications could have significant civil liberties implications.
In 2016, Congress granted the Defense Department power to trace, take control of or destroy drones within the United States, but the law limited that authority to three critical areas: protecting facilities involved with nuclear deterrence, space and missile defense.
Last year , the Trump administration sought broad counter-drone powers for federal agencies. That request foundered on bipartisan concerns in Congress that it was too expansive.
Under the legislation Trump signed last month, the Defense Department's powers were expanded significantly. There are six new areas where it can track or take down drones, including working to protect the president, vice president "or other officer immediately next in order of succession."
Air defenses, including in Washington, Special Operations forces activities, and certain combat support, testing and explosives facilities also were included.
The military's sense of urgency is due, in part, to its experiences using drones to deadly effect overseas and facing off-the-shelf drones on the battlefield. But its growing role in domestic drone defense is an important and little-debated shift.
The Department of Homeland Security says that "without statutory relief we remain constrained in responding" to the threat of drones and must rely on "conventional means." Without legal changes, the agency is "limited in its ability to fully develop counter UAS technologies — further delaying our security response," spokeswoman Anna Franko said.
Some detection systems are deployed in the Washington region, although coverage is limited. Homeland Security officials say they cannot discuss all that is being done.
Security concerns, including from the FBI, have held up regulations that would allow much broader use of drones for business.
A Trump administration pilot program on expanding drone use is meant, in part, to provide the data and experience to help assuage such concerns, backers said.
Fear of a surgical strike
In the Washington region, drone policing can be an absurd and disturbing affair, as efforts to deal with brazen and sometimes comical behavior are colored by a post-9/11 sense that potent attacks could come at any moment.
One man was detained in 2015 for flying his drone inside the chamber of the Jefferson Memorial. After allowing authorities to search his iPad controller and car, he was deemed "negative for any suspicious indicators" and given a ticket, according to an FAA incident database. In general, flying drones in a national park, without a special permit, is illegal.
In another instance, two Ukrainians were questioned by the Secret Service and had their drone confiscated after flying it near the Washington Monument, according to the database. Someone else was cited for filming a fitness video there.
But many operators remain out of law enforcement's reach, their identities and motives obscured.
On a late night in June, four drones were captured on security cameras flying across from the Pentagon, according to an incident report.
"One of the drones possibly crashed or burned in the sky and that was the last sighting," the report said. An Arlington County police spokeswoman said its officers responded but found nothing.
Duggan's effort to quantify the threat was unusual. Rather than relying on anecdotal information or random sightings, the commander of Joint Base Myer-Henderson Hall wanted data.
Duggan worked with a San Francisco-based detection company, Dedrone, run by a former German drone maker. Joerg Lamprecht, the company's chief executive, saw a business opportunity in late 2013, when a protester crashed a drone a few feet from Chancellor Angela Merkel.
"You have a lock in your home. It's not for your mailman or your neighbors. It's for the one bad guy who might sneak in," Lamprecht said. "It's the same for the airspace."
The company's radio-frequency sensor on the roof of the National Defense University at Fort McNair, along Washington's riverfront, documented 52 drone sightings in 26 days. Technicians then moved the equipment across the Potomac to the fitness center at Fort Myer, near Arlington National Cemetery in Virginia. They tallied another 43 sightings in a month. Some drones appeared multiple times.
The sensors generally pick up signals within a kilometer or two, sometimes farther, the company said.
The areas covered reach far beyond the posts themselves but represent just a fraction of the federal no-fly area for drones. Washington's Flight Restricted Zone stretches about 15 miles from Reagan National Airport and bans drone flights not specifically authorized by the FAA. Such permissions are exceedingly rare, and none were given near the Army posts at the time, the agency said.
Given the technology used, the company couldn't provide precise locations for the drones or operators, although that can be done with other equipment.
The detection effort could also have picked up signals from the federal government's own drone research.
As a career Special Forces officer with experience tracking cyber- and other "asymmetrical" attacks, Duggan has for years studied the mind-set of adversaries seeking ways to inflict the greatest harm on the United States at the lowest cost. A drone attack is one of many, he said.
Given that Russia-aligned forces are believed to have used a drone to drop a grenade on a large ammunition depot in Ukraine last year , disrupting supply lines, and that the Islamic State arms cheap commercial drones in Syria and elsewhere, the risks weigh on him.
Surrounded by the history of Fort Myer, along the path taken by the mounted platoon that carries flag-draped coffins to Arlington, Duggan considered a Parrot Bebop drone that has been used in Syria and popped up repeatedly in the detection data. On Sept. 3, the aircraft was detected multiple times after 1 a.m. Then it returned for 21 minutes the next evening.
"It was just odd. It was atypical," Duggan said.
It probably wasn't probing his defenses or doing surveillance, he said. But a determined foe could do that — or worse, perhaps, with a "specific surgical strike or just to paralyze or cause fear," undermining readiness.
"Bases are not sanctuaries," Duggan said. "If I'm an adversary, this is where I'm going to take you out. Why? Because we have this mentality that we're all safe, everything's good."