London Gatwick operations were interrupted by at least two drones, prompting suspension of services, during a serious incursion on the airport last year, a police probe has determined.

Sussex county’s police force says the “deliberate criminal act” closed the airport for 30h over 19-21 December, causing disruption to 1,000 flights and over 140,000 passengers during a peak holiday period.

No-one has yet been convicted for the attack.

There were 129 separate sightings of drone activity, the investigation found, of which 109 were from “credible witnesses” – such as pilots, airport personnel and police – who were used to working in the Gatwick environment.

“Through corroborated witness statements, it is established that at least two drones were in operation during [the attack],” it adds. “The offender, or multiple offenders, had detailed knowledge of the airport.”

Drone activity occurred in groupings on 12 separate occasions individually lasting from 7min to 45min.

“On six of these occasions, witnesses clearly saw two drones operating simultaneously,” the probe states.

Ninety-six persons of interest have been identified but ruled out, and Sussex Police assistant chief constable Dave Miller admits that an “exhaustive” investigation has yet to find those responsible.

“Without new information coming to light, there are no further realistic lines of enquiry at this time,” he says. There is no evidence of a terrorism connection or that the attack was led by a campaign group.

Sussex Police puts the cost of the operation and inquiry at £790,000 ($974,000).

“Equipment was quickly installed using both military and private assets to bring [the drone incursion] to a conclusion and allow the airport to re-open,” says Miller.

He describes the situation as “unprecedented”, occurring at a time when the UK government was starting to examine drone countermeasures.

“Measures now available have strengthened our capability to respond to and investigate a similar incident in the future,” he adds.