Police hunting those responsible for the Gatwick drones chaos have identified "persons of interest", the BBC understands.
The UK's second busiest airport has reopened after drones flying over the airfield closed it for more than a day.
Police are exploring "a number of lines of inquiry", including environmental activism and "high-end criminal behaviour".
Officers are focusing on "likely locations in and around the airport".
They said there was "no evidence" the use of the drones was state-sponsored.
Gatwick's runway reopened on Friday morning and 837 flights were scheduled, but there were still delays and cancellations.
Thousands of passengers - including some attending funerals, honeymoons and Christmas reunions with their families - remain stranded at Gatwick.
Police pursuing “significant” lines of enquiry re Gatwick drones, and have identified persons of interest - no arrests or interviews yet though, @ACCSteveBarry tells me— Ben Weisz (@BenRTWeisz) December 21, 2018
Steve Barry, Sussex Police Assistant Chief Constable, told the BBC "persons of interest" had been identified as part of the investigation but officers were keeping an open mind over the motive.
While terrorism has been ruled out, environmental activism is one line of inquiry.
Mr Barry said there was no evidence of involvement of a foreign power, but described the drone activity as "really high-end criminal behaviour".
"This is a really significant criminal offence," he said.
"We're working on the assumption that there was more than one drone operating around Gatwick in the last 48 hours.
"In terms of how many perpetrators, there's a number of lines of inquiry, there's an ongoing investigation, we're pursuing that trying to find out who has been responsible for this really malicious criminal behaviour."
Mr Barry said a drone had last been seen at 22:00 GMT on Thursday.
He added measures to tackle the threat ranged from "technical, sophisticated" options to "detect and mitigate drone incursions" right through to "less sophisticated options", including using shotguns.
He said the drones could have been operated from a fair distance away, but police were focusing on "likely locations in and around the airport".
Asked when measures were first put in place, Mr Barry said: "There were always some measures in place. The additions to those took some time to request and to arrive here at Gatwick."
He said it took "hours rather than days" to request extra measures, but added: "Coordinating that, deploying that, getting it set up at Gatwick has taken some time but we've learnt from that."
"This incident has been really unusual in the concerted and malicious and really criminal level of the behaviour of the drone operator and that did cause us some challenges," he added.
Gatwick boss Chris Woodroofe said "mitigating measures" from the government and military had given him "confidence to reopen the airport".
The airport is expected to be "back to normal" by the end of Saturday.
It could not operate while the drones were in flight in case they hit and damaged a plane.
Pilots' union Balpa said it understood detection and tracking equipment had been installed around Gatwick's perimeter and that if the drones reappeared the airport would close again.
One family said their Christmas holiday plans had been ruined.
Kevin and Lisa Haynes, and their daughters Aphea, eight, and 11-year-old Jayda, were due to go on a cruise, but their flight to Madrid was cancelled, which meant they would miss their connecting plane to Miami.
"What can you do about it?" Mr Haynes, from Wolverhampton, said. "You can understand the airport's view. They've got to ensure health and safety."
Garron Akushie-Stevens, 26, said he was going to miss his uncle's wedding in Ghana.
He had been due to travel as part of a four-strong party for the wedding on Christmas Eve.
Mr Akushie-Stevens, from Neasden, north-west London, said: "He's a close uncle to me.
"It's unacceptable. I feel traumatised, I feel let down and as if I've been thrown to the wolves."
'Medicine running out'
Ruth Knudsen, 63, from Hythe, in Kent, has been trying to get back to the UK with a "limited supply of chemo medication" - she said she was worried she would run out of medicine.
Her Gatwick-bound flight from Tenerife South should have left on Thursday. However, after a six-and-a-half hour wait she was told to retrieve her bags and join a queue to rebook her flight.
She said she had been given two options. One was to return to Gatwick on Sunday. The other was to return to Luton or Southend on Saturday.
She said: "I chose Luton as I could not risk rebooking to go to Gatwick in case the airport was not operational still."
It is illegal to fly a drone within 1km of an airport or airfield boundary and flying above 400ft (120m) - which increases the risk of a collision with a manned aircraft - is also banned.
Endangering the safety of an aircraft is a criminal offence which can carry a prison sentence of five years.