Sussex Police Chief Giles York told BBC radio that police have searched 26 potential launch sites near the airport but do not believe they have found the drone that was seen near the runway on Dec. 19 and Dec. 20.
York said he is "absolutely certain that there was a drone flying throughout the period that the airport was closed."
A senior detective said last week it was possible drones hadn't flown over the airport last week, sowing confusion, but police later insisted that the drone sightings were authentic.
The airport's closure led to more than 100,000 people being stranded or delayed in the worst ever drone-related disruption at an international airport.
No one has been found responsible despite the deployment of military assets to track and deter drone flights.
Two people — Paul Gait and Elaine Kirk — living just minutes from Gatwick Airport were arrested on suspicion of criminal drone use but were set free two days later when police said they were no longer suspects.
After his release, Gait complained bitterly about the police action — and the newspapers that published front page photos of the couple. York used his radio interview to apologize for the distress caused, although he said the arrest was lawful.
"I am really sorry for what he went through, but the reason why we held him was so that we could dispel everything in the first instance," York said.
Police had earlier revealed that one damaged drone had been found near the airfield and was being tested for DNA, fingerprints and other clues.
The motive for the drone intrusions is not yet clear. Officials say there are no indications the incident was "terror related."
Officials have not disclosed what military equipment was put in place.
British officials say they are able to deploy the equipment at other U.K. airports. It is not clear whether counter-drone measures have been increased at Britain's other major airports.
Gatwick is located about 30 miles (48 kilometers) south of London. It handles roughly 43 million passengers a year.