The Secret Service found a "quadcopter" drone on the White House grounds. The Post’s Carol D. Leonnig explains how the White House is protected from aerial threats. (Jorge Ribas/The Washington Post)

The intrusion by a recreational drone early Monday onto the White House lawn exposed a security gap at the compound that the Secret Service has spent years studying but has so far been unable to fix, according to several officials familiar with the concern.

The episode came just four days after lawmakers examining White House security had been warned by a panel of experts that the Secret Service’s inability to identify and disable drones remained one of the leading vulnerabilities at the complex, according to two people with knowledge of the discussions.

Just after 3 a.m. Monday, the drone flew at a low altitude over the south grounds of the White House without setting off alarms and was spotted by a Secret Service officer standing guard.

The “quadcopter” device, approximately two feet in diameter, then crashed on the southeast corner of the property, prompting a lockdown at the complex until the device was examined and determined not to pose a danger. Officers and agents with flashlights scoured the complex and surrounding area for clues.

A man called authorities roughly six hours later to report that he was a recreational operator and had mistakenly crashed the drone. He said he did not mean to fly it onto the White House grounds, according to the Secret Service.

A small ”quadcopter” crashed onto the White House grounds early Monday, officials said.

Monday’s incident was believed to be the first time that a drone had penetrated the White House perimeter, an especially sensitive threshold that immediately caught the attention of lawmakers and counterterrorism officials.

The incident brought public attention to what has been an ongoing source of anxiety within the agency tasked with protecting the White House and, more broadly, among government officials in charge of protecting nuclear power plants, military bases and other sensitive areas seen as vulnerable to potential attack from drones.

Most recreational drones, like the one that crashed Monday, weigh only a few pounds and lack the power to do much harm. But Department of Homeland Security and Secret Service officials have studied the potential that these devices, which can be purchased cheaply and easily, could be modified to carry explosives or weaponry.

Larger models that can carry payloads of up to 30 pounds are available on the market and are expected to become more common.

Small drones have violated the highly restricted airspace near the White House and the Capitol before without attracting public attention.

According to Federal Aviation Administration records, authorities arrested an unidentified man on Aug. 19 after he crashed a drone into a tree in Freedom Plaza, just east of the White House. On July 3, a Secret Service patrol detained a person who was flying a quadcopter about one block from the White House grounds. People have also been nabbed in recent months for flying drones near the Capitol and the Lincoln Memorial.

At the White House, a proposal for a higher fence, along with already added surveillance cameras and sophisticated environmental sensors, has been aimed at making the presidential residence more secure in the wake of a series of security lapses that have proved embarrassing for the Secret Service, including an intruder making it deep into the house in September. And antiaircraft missiles have been in place for years to guard against airplane attacks.

But officials said the Secret Service does not have the ability to easily identify and stop a typical drone.

Most models are too small to appear on radar and are not equipped with electronic transmitters that broadcast their location.

Also, federal law prohibits the use of any device that could be used to jam or interfere with the GPS signals that drones rely on for navigation.

Unlike regular aircraft, drones used for noncommercial purposes are not affixed with registration numbers and pilots are not required to obtain a license. As a result, it is extremely difficult to track down the owner of a rogue drone unless the operator is fiddling with the remote controls in plain sight.

One official familiar with internal discussions said the Secret Service is “still trying to work through this, how to defend against them.”

Secret Service spokesman Brian Leary declined to comment.

Tracking drones

Frederick F. Roggero, a retired Air Force major general who heads an aviation safety and risk-management consulting firm, Resilient Solutions, said his company has been in talks with several government agencies about finding ways to track rogue drones.

He said commercial technology is available that can use a combination of sensitive radar and acoustic trackers to detect small drones, though coming up with an effective way to stop them has been more elusive.

“To do something about the problem, you have to find it, you have to track it, you have to identify it and you have to decide what to do with it,” Roggero said. “But especially in an urban environment, it would be tough to detect and tough to defeat kinetically without shooting it down and causing collateral damage.”

Rep. Elijah E. Cummings (Md.), the ranking Democrat on the House Oversight and Government Reform Committee, said he has repeatedly asked the Secret Service whether it has deployed the most cutting-edge technology in its efforts to safeguard the president and its other protectees. Cummings pointed to the mobile air-defense system erected by Israel to intercept and shoot down enemy missiles before they strike targets in the country.

“Just like our friends in Israel feel comfortable with that ‘Iron Dome,’ I want the people in the White House to feel comfortable, too, and I want the people who are trying to do us harm to know they cannot penetrate that sky over the White House,” Cummings said.

Rep. Jason Chaffetz (R-Utah), chairman of the Oversight Committee, said Monday morning’s incident is “deeply concerning” because it lays bare the chink in the White House armor that drones can get past.

“These kinds of threats are not going away,” Chaffetz said, adding that he believes the Secret Service is working hard on the problem and is “on top of it.”

Rapid technological advances have made small drones increasingly affordable, easy to fly and popular. By some estimates, manufacturers are selling as many as 15,000 consumer models each month in the United States. Developing reliable defenses to protect sensitive areas from drones, however, has posed a challenge.

Brian Hearing, co-founder of Drone Shield, a Washington-based company that helps private clients detect the presence of drones, said many people in the industry had been bracing for an event like Monday’s.

“We always suspected that a high-profile event like this at the White House would occur and have been a little surprised that some protest group hadn’t already done something like it,” he said.

The flood of inexpensive surveillance drones on the market has already spurred demand for countermeasures from people worried about their privacy. Drone Shield, founded 18 months ago, has built a thriving business by providing detection systems to clients, including Hollywood celebrities looking to fend off paparazzi stalking them with camera-equipped drones.

“That’s our bread and butter,” said Hearing, Drone Shield’s co-founder. “There’s been a higher demand than we anticipated, but nobody could have anticipated how quickly the drone industry would have expanded.”

Hearing said his firm’s technology can detect drones and their flight path by sensing their acoustic signature. An alert or alarm is sounded to make clients aware of the presence of a drone and give them time to scurry indoors. The system, however, cannot do anything to stop a drone in midair.

Terrorism fears

U.S. counterterrorism officials have long worried about the potential that terrorists might use small drones in an attack.

In 2012, a Massachusetts man received a 17-year prison sentence after he pleaded guilty in a plot to attack the Capitol and the Pentagon with drones carrying plastic explosives.

Rezwan Ferdaus, who studied physics at Northeastern University in Boston, built detonators that he planned to trigger with cellphones, according to federal prosecutors.

In November, French security officials reported a spate of incidents in which small drones were seen conducting surveillance over 13 nuclear power plants. Fighters with the Islamic State have also used commercially available drones to conduct surveillance of Syrian military targets and have posted some videos of their efforts on the Internet.

On Jan. 16, officials from the Department of Homeland Security sponsored an invitation-only conference in Northern Virginia to discuss the potential threats that drones pose to sensitive public installations. According to participants at the conference, officials from DHS, the Defense Department, the FAA and the National Counterterrorism Center all gave presentations on an array of federal efforts underway to counter threats from small drones.

The FAA imposes strict safety regulations on drones flown by government agencies or anyone who operates them for commercial purposes. In contrast, hardly any rules apply to people who fly drones as a hobby, other than FAA guidelines that advise them to keep the aircraft below 400 feet and five miles from an airport.

Sen. Charles E. Schumer (D-N.Y.) said Monday’s incident at the White House should spur safety rules for drone operators.

“With the discovery of an unauthorized drone on the White House lawn, the eagle has crash-landed in Washington; there is no stronger sign that clear FAA guidelines for drones are needed,” he said in a statement.