Shooting down a drone which has caused chaos at Gatwick Airport is a “tactical option” being considered by police.
The measure had previously been ruled out by Sussex Police, which cited concerns over “stray bullets”.
But with the airport expected to remain closed on Friday, Det Ch Supt Jason Tingley said the force would “do what we can to take that drone out of the sky”.
The Army has been deployed to assist the police operation.
Gatwick’s chief operating officer Chris Woodroofe said 120,000 passengers had been due to fly out since the airport closed its runway on Wednesday.
“We will review overnight whether there is any potential to open tomorrow,” he said.
“But we are working up contingency plans all the way through to no flights tomorrow.”
‘Closed for the foreseeable’
Speaking at 21:30 GMT, he said the situation remained “fluid”, as the drone had been spotted again less than an hour earlier.
Police have been locked in a game of cat and mouse with the drone, and have received about 50 reports of the device being flown near the airfield since Wednesday evening,
No arrests have been made, and the airport is expected to be closed for the “foreseeable future” while the hunt for the operator continues.
Those due to travel on Friday have been urged to check with their airline before arriving at the airport.
Det Ch Supt Tingley said police did not know the make and model of the drone, but suspected it had been “adapted and developed” with the intent to cause disruption.
Officers had, however, deduced “certain specifications” such as the device’s size, he said, and urged anyone with pictures of the drone to contact police.
After other strategies to bring down the drone failed, Det Ch Supt Tingley said, the force was reconsidering plans to shoot it down.
He said police were also following lines of inquiry, including “particular groups” and a “number of persons of interest”.
Defence Secretary Gavin Williamson confirmed the military deployment on Thursday evening.
He said the armed forces had a “range of unique capabilities” and would “do everything we can so that they are in a position to open the airport at the earliest opportunity”.
Men in military uniform were seen arriving at the airport at about 20:00 GMT and liaising with police officers, before entering the grounds in convoy.
Transport Secretary Chris Grayling said: “We’ll do everything we can to ensure that if Gatwick is not open again quickly that we can get [passengers] away from other airports.”
He could not confirm whether the perpetrators were close to being caught but added: “There’s a huge amount of effort going on – we’ve got up-to-date technology, we’ve brought special technology into Gatwick to try and track this down.”
The shutdown started just after 21:00 on Wednesday when two drones were spotted flying “over the perimeter fence and into where the runway operates from”.
The runway briefly reopened at 03:01 on Thursday but was closed again about 45 minutes later due to “a further sighting of drones”.
Drones continued to be spotted throughout the day.
The police operation
More than 20 police units from two forces are searching for the perpetrator, who could face up to five years in jail.
Supt Justin Burtenshaw, head of armed policing for Sussex and Surrey, described attempts to catch whoever was controlling the drones as “painstaking” because it was “a difficult and challenging thing to locate them”.
“Each time we believe we get close to the operator, the drone disappears; when we look to reopen the airfield, the drone reappears,” he said.
As a result of an appeal for information, Sussex Police said it had been inundated with calls, but urged people to contact with information “focussed on the identity or location of the drone operator”.
How have passengers been affected?
About 10,000 passengers were affected overnight on Wednesday and Gatwick said 110,000 people were due to either take off or land at the airport on Thursday.
Incoming planes were diverted to other airports including London Heathrow, Luton, Birmingham, Manchester, Cardiff, Glasgow, Paris and Amsterdam.
Crowds of travellers spent the morning waiting inside Gatwick’s terminal for updates, while others reported being stuck on grounded planes for hours.
Some who spoke to the BBC included a couple hoping to honeymoon in New York and a seven-year-old who had been due to fly to Lapland.
Geoffrey Grove, 42, said he was stranded on a plane at Orly airport in France after his flight from Boston, USA, was grounded.
Passengers were stuck on board for hours and prevented from disembarking, he said.
“There is no air conditioning. Babies are being stripped because it is so hot,” Mr Grove said.
A Gatwick spokeswoman said extra staff had been brought in and they were “trying their best” to provide food and water to those who needed it, and they would be leaving the heating on overnight for stranded passengers.
The Civil Aviation Authority said it considered this event to be an “extraordinary circumstance”, and therefore airlines were not obligated to pay any financial compensation to passengers.
Alex Neill, from consumer rights group Which?, said people “may still be entitled to meals, refreshments, hotel accommodation or transfers”.
Airports and drones: The law
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.
The number of aircraft incidents involving drones has grown dramatically in the past few years, as the popularity of the devices has increased.
In 2013 there were zero incidents, compared to almost 100 last year.
Mr Grayling said the events at Gatwick are “not something that’s not been experienced in this country before”, even though drones have been a problem elsewhere in the world.
The government was now looking to “go further” with drone-control, he said, including considering age-limits for users.
He added: “Anyone who tries to do the same [as at Gatwick] again, should expect to go to jail for a long time.”
The UK Airprox Board assesses incidents involving drones and keeps a log of all reports.
In one incident last year, for example, a pilot flying over Manchester saw a red “football-sized” drone passing down the left hand side of the aircraft.
In another, a plane leaving Glasgow narrowly missed a drone. The pilot in that case said the crew only had three seconds of warning and there was “no time to take avoiding action”.
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