London's Gatwick Airport was operating without problems Sunday, but the fugitive drone operators who brought incoming and outgoing flights to a standstill over multiple days remained at large — and a potential threat — after police cleared two local residents who were arrested as suspects.

Paul Gait, 47, and his wife Elaine Kirk, 54, were detained on Friday night on suspicion of flying a drone above Britain's second busiest airport.

Sussex Police were hopeful they had halted the disruptive and costly drone incursions during one of the heaviest travel periods of the year with Friday's arrests of a couple who live near the airport. But they were released Sunday, and police said they were no longer suspects.

Elaine Kirk and Paul Gait have been released without charge. Photo / Supplied

UK correspondent Gavin Grey told Tim Dower people believe the police have wasted time.

"Friends are saying, if police had just checked with us we could have told them that at the point where the drone was meant to be flying over Gatwick Airport, we were with them, they were at work."

Tens of thousands of passengers suffered through long flight delays or were stranded by cancellations after two drones first were spotted above the airfield at Gatwick on Wednesday night, prompting an immediate suspension of all air traffic.

Airport authorities consider drones a menace because they could damage a plane in flight or be sucked into a plane's engine, causing a deadly crash. Police told Sky News Sunday that a damaged drone had been found near Gatwick and may provide useful forensic clues.

After the shutdown extended into Thursday, increased military protection was brought in Thursday night to watch for more drones while planes resumed taking off and landing at Britain's second-busiest airport.

Grey says now police are back at square one trying to find out who's responsible.

"Of course, the fear now is if it's not them, who was it, do they still have the drone, will they do it again, and why exactly are they doing it?"

The government has kept the details of the security operation secret, but the military equipment is thought to offer better tracking capabilities and give authorities early warning if drones near Gatwick, located 30 miles (45 kilometres) south of London.

At the airport on Sunday, flight arrival and departure boards showed fewer delays than on Saturday. Additional tracking gear at the periphery of the runway and an increased police presence were the only clear signs of the headaches experienced there in the past four days. But officials have stepped up surveillance of the surrounding airspace behind-the-scenes.

British Transport Secretary Chris Grayling has said the unspecified "military measures" have bolstered confidence that the airport will be able to remain open without further interruptions. But anti-drone technology is a relatively new, imperfect field, Grayling said.

It is not clear how officials will respond if another drone gets too close. The best guide may be what happened on Friday night, when the airport was shut down for 70 minutes after a drone was spotted. The military deployment allowed the airport to reopen relatively quickly, authorities have said.

No confirmed drone incursions have taken place since then — a factor that led many to assume police had found the responsible operators when they arrested the two suspects Friday night.

That hope ended Sunday when the couple — a 47-year-old man and a 54-year-old woman — was allowed to return to their home in Crawley, a 5-minute drive from Gatwick.

Sussex Chief Detective Jason Tingley said he' was satisfied the two were not involved. He is hoping a tip will provide a crucial lead.

"We ask for the public's continued support by reporting anything suspicious, contacting us with any information in relation to the drone incidents at Gatwick," he said.

The drone attack causes chaos at the airport, wrecking Christmas travel plans for hundreds of thousands of passengers. Photo / AP

Tingley said police did not identify the suspects but The Mail on Sunday tabloid named them and published a large front-page photo of them with the headline, "Are These the Morons Who Ruined Christmas?"

The crisis at Gatwick marked the first time drones caused sustained disruption at a major airport. There is not much data on the dangers drones pose to airplanes because they are a relatively new phenomenon.

Police say the motive for the drone incursion is not yet known but they do not believe it is "terror-related."

Gatwick Airport, which handles roughly 43 million passengers per year, has offered a 50,000 pound ($63,000) reward through Crimestoppers for information leading to the arrest and conviction of the drone operators.