Image copyright Getty Images Image caption The travel plans of about 140,000 people were disrupted as a result of the drone attack

The drone attack that caused chaos at Gatwick before Christmas was carried out by someone with knowledge of the airport's operational procedures, the airport has said.

A Gatwick chief told BBC Panorama the drone's pilot "seemed to be able to see what was happening on the runway".

Sussex Police told the programme the possibility an "insider" was involved was a "credible line" of inquiry.

About 140,000 passengers were caught up in the disruption.

The runway at the UK's second busiest airport was closed for 33 hours between 19 and 21 December last year - causing about 1,000 flights to be cancelled or delayed.

In his first interview since the incident, Gatwick's chief operating officer, Chris Woodroofe, told Panorama: "It was clear that the drone operators had a link into what was going on at the airport."

Image caption Chris Woodroofe led Gatwick's response to the attack

Mr Woodroofe, who was the executive overseeing the airport's response to the attack - the "gold commander" - also said that whoever was piloting the drone could either see what was happening on the runway, or was following the airport's actions by eavesdropping on radio or internet communications.

And whoever was responsible for the attack had "specifically selected" a drone which could not be seen by the DJI Aeroscope drone detection system that the airport was testing at the time, he added.

'No overreaction'

Despite a huge operation drawing resources from five other forces and a £50,000 reward, there is still no trace of the culprit.

Sussex Police says its investigation is ongoing and expected to take "some months to complete".

The first sighting of the drone was at 21:03 GMT on 19 December but it was not until 05:57 GMT on 21 December that flights resumed with an aircraft landing.

Gatwick says it repeatedly tried to reopen the runway but on each occasion the drone reappeared.

Airport protocol mandates that the runway be closed if a drone is present.

Image copyright Getty Images Image caption The military deployed equipment at the airport after the drone sightings

Mr Woodroofe denied claims the airport overreacted, describing the situation it faced as an unprecedented, "malicious" and "criminal" incident.

"There is absolutely nothing that I would do differently when I look back at the incident, because ultimately, my number one priority has to be to maintain the safety of our passengers, and that's what we did.

"It was terrible that 140,000 people's journeys were disrupted - but everyone was safe."

Mr Woodroofe also dismissed the suggestion that the number of sightings had been exaggerated - and a theory, circulating online, that there had been no drone at all.

These claims have been fuelled by the fact that there are no verified pictures of the drone, and very few eyewitnesses have spoken publicly.

Police told the BBC they had recorded 130 separate credible drone sightings by a total of 115 people, all but six of whom were professionals, including police officers, security personnel, air traffic control staff and pilots.

Image copyright Getty Images Image caption Gatwick's runway was shut for 33 hours

Image copyright Getty Images Image caption About 1,000 flights were cancelled or delayed

Image copyright Getty Images Image caption The runway reopened on the morning of 21 December

Mr Woodroofe said that many of the drone sightings were by people he knew personally and trusted - "members of my team, people I have worked with for a decade, people who have worked for thirty years on the airfield, who fully understand the implications of reporting a drone sighting".

"They knew they'd seen a drone. I know they saw a drone. We appropriately closed the airport."

Panorama has been told witnesses reported seeing an extremely fast-moving, large drone with bright lights.

At least one person noted the characteristic cross shape while others described it as "industrial or commercial" and "not something you could pop into Argos for", an airport spokesperson said.

Threat remains

Other international airports have installed counter-drone technology and Gatwick has confirmed that, in the days after the attack, it spent £5m on similar equipment.

Asked whether Gatwick should have done more to protect the airport from drones before the incident, Mr Woodroofe said the government had not approved any equipment for drone detection at that stage.

"The equipment I have on site today is painted sand yellow because it comes straight from the military environment," he added.

Panorama has learned that Gatwick bought two sets of the AUDS (Anti-UAV Defence System) anti-drone system made by a consortium of three British companies.

AUDS was one of two systems the military deployed at the airport on the evening of 20 December.

Image caption The AUDS anti-drone system at work

Image caption Gatwick has purchased new equipment since the disruption

Mr Woodroofe said he was confident that the airport was now much better protected.

"We would know the drone was arriving on site and we'd know where that drone had come from, where it was going to, and we'd have a much better chance of catching the perpetrator."

Every day, he said, the airport sends up a drone to test the detection equipment, and "it finds that drone".

But he added: "What this incident has demonstrated is that a drone operator with malicious intent can cause serious disruption to airport operations.

"And it's clear that disruption could be carried over into other industries and other environments."

Panorama, The Gatwick Drone Attack, will be shown on BBC One at 20:30 BST on Monday 15 April and on BBC iPlayer It will also be shown on BBC World News at a later date