Police are still looking for whoever used a drone to shut down Gatwick Airport for more than 32 hours.

The airport has reopened, but officials warn that if the drone comes back they will close the runway again.

Police say they are pursuing leads and now have "options" it the drone comes back. They did not say what those options are.

More than 120,000 people have had their flights canceled, rerouted, or delayed.

Police have still not caught whoever used a drone to shut down London's Gatwick Airport for more than 32 hours —and officials admit that the pilot could shut down the airport all over again by simply flying it back.

Officers first started trying to find the drone pilot on Wednesday evening, after Gatwick officials spotted the device and shut down the runway.

As of midday on Friday, Sussex Police have spent 39 hours looking for the culprit. They have made no arrests and have not made public any details of who they believe it could have been.

More than 120,000 people had had their journeys disrupted in some way by the cancellation, which paralyzed the airport for 32 hours.

Passengers wait in Gatwick's South Terminal on Thursday. REUTERS/Peter Nicholls

This video visualization shows the disruption caused to flights when a drone was spotted near Gatwick's runway in 2017, forcing a similar closure:

The airport tentatively re-opened around 6 a.m. on Friday, but it could not guarantee that the drone won't just come back and begin the cycle of disruption again.

Gatwick's Chief Operating Officer, Chris Woodroofe, said on Friday that the runway will close again if the drone returns, The Guardian reported.

He said: "My number one priority is going to be the safety of our passengers. And so, if the drone comes and endangers an aircraft then we will suspend runway operations."

During their investigation, Sussex Police committed at least 20 units and a police helicopter to the hunt.

Photographs taken on Thursday show officers on rooftops around Gatwick, and patrol cars were circling the airport perimeter.

Police stand near equipment on the roof of a building near Gatwick Airport. Jack Taylor/Getty Images

Troops from the British Army were also deployed, though it is unclear whether they were anything to do with trying to catch the operator or whether they were focused on trying to take down the drone.

Read More: Video shows the drone that shut down a major British airport for more than 32 hours

Woodroofe told the BBC on Friday morning that the airport has put in additional measures to protect the airport if a drone appears. He did not say what the measures are.

He said he could not comment on whether the police would try to shoot down a drone if it reappeared. On Thursday, he said that police were refusing to shoot for fear that a stray bullet could hit somebody.

People wait near the departures gate at Gatwick airport on Thursday. AP Photo/Tim Ireland

He said that the departure of the 700 flights expected to fly from Gatwick today depends on whether there are any more sightings of a drone.

Police operation

Sussex Police Assistant Chief Constable Steve Barry told the BBC that the police are still investigating and "made a lot of progress overnight."

He said the police are pursuing leads, and claims officers are "in a much better position" than the day before. He did not offer any specifics.

Read More: Video shows the damage a drone can do to a plane, as Gatwick Airport shuts down over rogue drones on the runway

The UK's Transport Secretary, Chris Grayling, told BBC Breakfast on Friday: "This kind of incident is unprecedented anywhere in the world, the disruption of an airport in this way."

"We're quite positive in terms of the way we are progressing the investigation."

Disrupting an airport with a drone is a crime in many places, including in the UK, where it can be punished with five years in prison.

Grayling said that whoever is responsible must "go to jail for a long time."