LONDON — British police insisted Monday that last week's drone sightings merit an ongoing investigation after questions were raised about whether the devices, which prompted a shutdown of the country's second-largest airport at the height of the Christmas travel season, ever existed at all.

Drones were spotted at Gatwick Airport on Wednesday, leading to a 36-hour closure that affected more than 120,000 passengers. Another sighting on Friday led to a second brief shutdown.

The airport was operating normally Monday, but military equipment remains in place to deter and track any fresh incursions.

Two people from a town near the airport were arrested Friday night in connection with the drone sightings but were released Sunday without charges. No other suspects have been identified, and the hunt for the culprits is ongoing.

But Sussex police chief Jason Tingley said Sunday it was possible that those who reported seeing drones after the first sightings raised alarms were mistaken.

"Of course, that's a possibility. We are working with human beings saying they have seen something," Tingley told the BBC, prompting widespread consternation.

Police issued further statements on Monday in an apparent attempt to clarify the situation.

Officials received 67 reports from the public — including airport staff and passengers — of drone sightings between Wednesday and Friday last week, the statement said, and a forensic examination of a damaged drone found near the airport was underway.

“We are interviewing those who have reported these sightings, [and] are carrying out extensive house to house enquiries," Tingley said in the statement.

#GatwickDrones | We can unequivocally state between 19-21 Dec there have been numerous #drone sightings at @Gatwick_Airport. We want to bring those responsible for the disruption to justice and through @CrimestoppersUK there is a £50,000 reward https://t.co/U3cQKvgaMO pic.twitter.com/bx5dqW67wT — Sussex Police (@sussex_police) December 24, 2018

The airport is now offering a reward of over $60,000 for information leading to the arrest and conviction of those responsible.

The closure of the airport was a safety measure, with authorities concerned the drones could disrupt airplane flight paths, disable jet engines or worse.

The drone crisis at Gatwick, which is located 30 miles south of London and serves 46 million passengers a year, has had ripple effects throughout the international air travel system.