Police investigating the chaos and disruption at Gatwick Airport have admitted there may never have been a drone at all.
More than 140,000 passengers were affected by last week's shutdown of Britain's second busiest airport, which was ordered after almost 70 drone sightings were reported in the space of a few hours.
But after two people were released without charge following three days in custody, a senior Sussex detective said it was possible the sightings had simply been false alarms but denied the investigation was "back to square one".
Detective Chief Superintendent Jason Tingley revealed that the force had received 67 calls from police officers, airport staff and members of the public reporting drone sightings, but said none of them had actually been confirmed.
Asked about speculation there was never a drone, he replied: "Of course, that's a possibility. We are working with human beings saying they have seen something."
Window fitter, Paul Gait, 47, and his wife, Elaine Kirk, 54, were arrested on Friday night on suspicion of being behind what police had previously said was a "deliberate act of disruption".
Police questioned them for 36 hours despite Gait having an alibi which placed him at work through most of the disruption.
Police have confirmed that the couple had been arrested following a tip off from a member of the public.
Tingley refused to apologise over the arrests but thanked the couple for their cooperation.
He went on: "We have put a lot of effort into supporting then when they were released from questioning."
With no obvious breakthrough in the police investigation the Crimestoppers charity put up a £50,000 ($93,900) reward for information leading the capture of those responsible.
The reward is in addition to the £10,000 ( $18,800) being offered by the charity's chairman, Lord Ashcroft.
Yesterday detectives discovered a damaged drone close to the airport and tests were being carried out to ascertain if it is linked to the incident.
Defending the investigation Tingley said: "We are not back to square one...we have a number of lines of enquiry and persons of interest. We are still progressing those lines of enquiry including house to house enquiries around the sightings of the drone."
But he conceded that the wet weather might hamper the search for any forensic clues from the downed drone.
It is understood one theory now being explored is that any drone attack might have been the work of a former disgruntled Gatwick employee.
The cat and mouse nature of the suspected culprit's movements on Thursday suggested someone with a good knowledge of the airport's layout.
A Whitehall source said: "Because of the way they were flying the drone, they had a lot of knowledge about the airport, how it worked and where everything was. It could be a former employee or someone who has done a thorough investigation into the airport."
Police have also refused to rule out that the culprit could strike again at another airpor