The advent of cheap, commercial drones has created the greatest explosion of pilots since the dawn of flight. For a few hundred dollars, anyone can pick up a flying robot and suddenly the security concerns begin. Most ineptly flown drones will find themselves in trees or rooftops or nearby streams, but sometimes the drones will fly too close to an airport. When that happens, flights into and out of the airport can be delayed, crewed aircraft pilots can start seeing drones in every shadow, and a poorly flown toy can disrupt the travel and safety of hundreds if not thousands.

Most famously, drone sightings shut down Gatwick Airport in the U.K. for 36 hours, stretching from Dec. 19 through Dec. 20, 2018. The sightings, and the panic that went with it, demonstrated three points that anyone doing security in 2019 should keep in mind: there simply aren’t good, readily available counterdrone tools yet; human eyesight is not particularly great at spotting small flying objects in the sky; and drones make disruption cheap for perpetrators and expensive for the organizations that are disrupted.

Besides the Gatwick example, conflicts across the globe have seen commercial drones converted into weapons of war and propaganda. There’s at least one reported case of a drone colliding with a passenger-carrying plane, though planes are still more likely to collide with birds than drones by several orders of magnitude. And now the ability of drones to interfere with airliners — and inconvenience their passengers — has been demonstrated on two continents. Law enforcement authorities are trying to figure out who flew a drone so high and so close to Newark Liberty International Airport Jan. 22 that incoming flights were held up briefly during a peak hour at one of the nation’s busiest airports.

In the Newark case, flights resumed within about 30 minutes — much more quickly than after a similar incident last month at London’s Gatwick Airport. However, understanding the risk posed to national security by consumer tech, the laws governing drone use and the ways in which drones can operate outside the parameters set by law is important both domestically and abroad. Drones are only getting cheaper, and there are more drone pilots every day.

Here are some common questions about these incidents and brief answers.

WHAT HAPPENED IN NEW JERSEY?

The pilots of both a Southwest Airlines flight and a United Airlines flight reported seeing a drone around 3,500 feet (1,000 meters) above Teterboro, New Jersey, about 9 miles (15 kilometers) from the Newark airport, on Tuesday.

As a precaution, the Federal Aviation Administration held up 43 flights already in the air and bound for Newark; nine landed instead at other airports. Another 170 Newark-bound planes were briefly delayed on the ground before taking off from other airports around the country.

No video of the reported drone has surfaced.

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WHO WAS OPERATING THE DRONE?

Authorities have not determined that. The FAA alerted New Jersey State Police and the FBI.

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CAN WE BE SURE THERE WAS A DRONE?

Some drone operators are skeptical about a drone reported at 3,500 feet and whether pilots in a fast-moving jet could accurately identify such a tiny object.

Vic Moss, a founder of Drone U, a drone-operator school based in Albuquerque, New Mexico, said many consumer drones are restricted from going that high, although home-built devices or older drones are not. There are, however, videos online showing drones at such altitudes.

"It's possible, but it's just incredibly unlikely that it was an actual drone," Moss said. "Drones are the new UFO."

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WHAT HAPPENED IN LONDON?

In mid-December, hundreds of flights were canceled and more than 100,000 people were stranded or delayed over two days after reports of drones spotted near the runway at Gatwick Airport, a major international hub.

A few days later, police arrested two men living near the airport but later cleared them, and no other suspects have been identified. Police also said that two drones found near the airport were not involved in the disruption.

A few weeks later, a reported drone sighting briefly halted flights departing from London's Heathrow Airport, one of the world's busiest.

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WHY IS THIS HAPPENING?

If the intrusions in New Jersey and London were deliberate, the motives are not clear. Officials in London said there was no indication that the Gatwick incident was terror-related. A criminal investigation has been opened into the Heathrow incident.

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WHAT ARE THE LAWS ABOUT FLYING DRONES NEAR AIRPORTS?

Federal rules forbid operating a drone within 5 miles (8 kilometers) of most airports or above 400 feet (120 meters) without a waiver from the FAA.

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ARE DRONE MANUFACTURERS RESPONSIBLE?

Devices from the biggest maker of consumer drones, DJI, include so-called geofencing — technology designed to prevent the aircraft from taking off near an airport. A drone that is launched properly but enters a no-fly zone will hover at the edge of the zone, according to a DJI spokesman.

Owners say DJI can take days to unlock no-fly restrictions around even small airports. But DJI says those requests now are automated and handled quickly.

However, many drones offered for sale don't include such restrictions: They have no GPS or geofencing.

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CAN OPERATORS DISABLE SAFETY SYSTEMS?

Yes. There are online discussions in which drone operators talk about hacks, but they involve some level of technological sophistication.

"The geofences (from manufacturers like DJI) are in place, but in some cases they can be defeated — it's not easy," said Tom Kilpatrick, a drone pilot who founded a drone company in Oklahoma. "They are designed to prevent the average drone operator from flying near an airport."

Home-built drones would likely not have those same safety features.

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WHAT'S BEING DONE TO PREVENT DRONES FROM INTERFERING?

DJI says it has developed technology to track nearby drones — their flight path and the operator's location — using mobile, ground-based units.

The technology is currently only used to identify other DJI drones.

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WHAT ARE AIRPORTS DOING?

The Port Authority of New York and New Jersey, which operates the Newark airport, said in a statement that agency officials met last week with counterparts from the FAA, FBI and Homeland Security Department "to review and enhance protocols for the rapid detection and interdiction of drones." A spokesman would not provide specifics and declined to say whether the airport has any anti-drone technology.

After the Gatwick incident, British officials said they have deployed drone-defense equipment at other U.K. airports, although they gave few details.

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ARE TOUGHER RULES IN THE WORKS?

Late last year, Congress gave the Homeland Security and Justice departments authority to develop and deploy a system to identify drones and disable — even destroy — drones that authorities consider a threat.

FAA spokesman Greg Martin said any such system has to be designed carefully so that it doesn't interfere with navigation equipment used by planes.

Associated Press journalist Julie Jacobson in New York and C4ISRNET staff writer Kelsey D. Atherton contributed to this report.