Two drones found near Gatwick Airport have been ruled out by investigators trying to discover who was behind the intrusion which disrupted hundreds of flights days before Christmas.
Sussex chief constable Giles York said officers had searched 26 potential drone launch sites, but do not believe they had found the one - or ones - flown near runways on December 19 and 20.
Major delays and cancellations continued for more than 24 hours after the drone was first spotted, affecting around 140,000 passengers.
Speaking to BBC Radio 4's Today programme, Mr York said he was "absolutely certain" there had been a drone flying throughout that period - but said he did not think detectives had found it yet.
Two drones had been found nearby and examined by police.
"I think the fact that we have found two drones so far as a result of this does show the extent of the search that has been carried out. I am led to believe that we are able to rules those drones out of this investigation at this time," he said.
Police received 115 reports of sightings in the area, including 92 confirmed as coming from "credible people", he said.
"Of course, we will have launched our own Sussex Police drones at the time with a view to investigate, with a view to engage, with a view to survey the area looking for the drone, so there could be some level of confusion there," he added.
He also confirmed that military technology was now in place at Gatwick, and though he declined to identify the nature of the equipment, he said the systems now in place were "dramatically different" to those in place a week ago.
Asked whether he could rule out a repeat of last week's disruption, Mr York said: "I don't think you can ever rule out anything happening again.
"But what we can say is what is at the heart of this is ensuring it is safe for the aircraft to take off and that's the different position that Gatwick Airport finds itself in today."