Mysterious drones have been swarming the night skies over New Jersey and other surrounding states for a month. They have been spotted over several US military sites. They were filmed over houses and apartment buildings. A flock was seen following a US Coast Guard rescue boat, while at the same time New Jersey police reported 50 drones arriving on land from the sea. But no one seems to know who is piloting it or whether it is a coordinated effort.

The incidents have drawn the attention of state governors and lawmakers as well as members of the U.S. Congress, and the FBI has launched an investigation and urged the public to report sightings.

Witnesses describe the drones as being as loud as lawnmowers, with some being nearly the size of a small car – significantly larger than a typical quadcopter or multirotor drone that anyone can buy. “These are not necessarily just small hobby unmanned aircraft systems that you can buy for $2,000,” said Daniel Gerstein of the RAND Corporation, a think tank in California. “These feel like they have a longer range and are more sophisticated than what you can get at a hobby store.”

Blurry nighttime videos have been popping up all over social media sharing drone sightings in states like New Jersey, Pennsylvania and New York, including one video showing drones over the Verrazzano Narrows Bridge in New York City. The Federal Aviation Administration imposed restrictions on drone flights over the Trump National Golf Club and the Picatinny Arsenal military base in New Jersey after reports of drone activity in both areas. The sightings coincide with other drone swarms that have recently appeared near British military bases where US Air Force squadrons operate.

On December 10, the House Homeland Security Committee held a hearing on drone threats with officials from the FBI, U.S. Customs and Border Protection, and the U.S. Department of Justice. Officials described recent sightings as a possible mix of rotary drones and fixed-wing drones, but were able to provide little information about what the drones do and who might be operating them. However, they said the drones did not yet pose a serious threat. In a separate Department of Homeland Security briefing, the agency told lawmakers that some sightings may have involved aircraft being mistaken for drones.

Ryan Herd, a New Jersey mayor, recounted ABC News Live that officials confirmed that they are not US military drones and that they are not operated by a US technology company.

Meanwhile, in the United Kingdom, Defense Secretary Vernon Coaker told Parliament last month that authorities were investigating several drone strikes that occurred near several British military bases starting November 20. These bases support U.S. Air Force squadrons flying fighter jets, bombers and support aircraft.

“The common theme in all of these cases is that no one has completely cracked the code on how to find, track and, if necessary, shoot down small drones,” says Arthur Holland Michel, a journalist and author who writes about drones. “The second common theme is that if the person piloting the drone is actively trying to evade identification, the challenge of countering that drone will skyrocket.”

Radar and other sensors can track drones, but it is “still not practical to cover every inch of the country with detection and tracking systems,” leaving authorities “completely blind to drones in most of our airspace.” says Michel. “In general, once a citizen has spotted a drone and reported it or filmed it with their phone, it is too late (to take early action),” he said.

Gerstein says there is some uncertainty about who among local law enforcement, state and federal agencies has primary authority and responsibility to address such drones. And even when that’s sorted out, it’s not easy to figure out the best way to deal with them.

There are many counter-drone measures in place to either shoot down drones directly – with missiles, lasers, bullets and even other drones – or take control of suspicious drones and force them to land using electronic warfare signals, Gerstein says. Such technologies have been widely used during the drone war in Ukraine, while U.S. Navy warships and other naval vessels have shot down dozens of drones that threatened shipping in the Red Sea region.

“When it comes to shooting down drones, the most effective measures are often the most dangerous,” says Michel. “We simply cannot allow law enforcement to fire powerful projectiles into the air or activate military-grade signal jammers every time a drone flies over New Jersey.”