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The army is helping police after watching a drone that suspended Tuesday flights at Heathrow Airport.
Scotland Yard said that a "full criminal investigation" had been opened on the incident – and that officers were among those who saw the drone.
Departures from the airport of West London were suspended for about an hour.
It comes after thousands of passengers were caught in a disruption at Gatwick Airport last month after reports of drones were reported.
Heathrow Airport, which also works with the meteorological police, said it was monitoring the situation and apologizing to passengers affected by the disruption.
Wednesday morning, the airport said that "the situation was back to normal".
Met Commander Stuart Cundy confirmed that military assistance had been provided but would not discuss tactics in detail.
& # 39; Extensive research & # 39;
He added that the observation of the drone had been reported shortly after 17:00 GMT. Departing flights were stopped as a precaution during initial investigations.
"We are doing extensive research in the Heathrow area to identify anyone who may be responsible for the operation of the drone," he said.
"The illegal use of drones on an aerodrome is extremely dangerous."
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Heathrow is one of the busiest airports in the world
Transportation Secretary Chris Grayling said during the incident, he was in contact with the airport about drone observation and had spoken to the Minister of the Interior and the Secretary of Defense.
The BBC cameraman, Martin Roberts, who works with drones, said he was driving on the Meath Heathrow airport around 17:45 GMT when he saw what he thought was a drone.
"I could see, I would say, about 300 feet tall, bright, fixed red and green fixed lights on the Harmondsworth area," he said.
"I could say that it was a drone – these things have pretty distinctive lights – not a helicopter."
Gatwick said last week that he had spent £ 5 million to prevent future attacks. Heathrow has also confirmed that it would buy drone protection systems.
And it was announced this week that the police would have new powers to fight against the illegal use of drones.
In light of the Heathrow incident, Secretary General of the British Airline Pilots' Association, Brian Strutton, called on ministers to strengthen drone legislation and ensure that airports invest in technologies protection.
"It is time to act quickly and decisively," he added.
Addressing ITV, Cabinet Office Minister David Lidington said, "It is clear that the government is reviewing the law to see if there are ways to strengthen it."
He added that airports must also "step up" their investment in technology to detect and prevent drones from flying.
BBC correspondent for transportation, Tom Burridge, said that if the "important" incident had not caused the same disruption as in Gatwick, he was asking the question of how airports British could handle drones.
While they are studying the latest technologies, it is clear that they are "catching up," he said, adding that the aviation industry was calling for "drastic measures." in this area for months ".
Alice Evans, BBC London, Heathrow
A few hours after the first report of a drone in the skies above Heathrow, everything was as usual in the departure lounge of Terminal 5. A low blow to Gatwick, a staff member said it was because "we are a good airport".
Although the flights were restored fairly quickly, many passengers were disrupted.
Catriona Walsh, who was flying from Basel, is one of those.
Ms. Walsh, who worked for two days while she was on maternity leave, stated that her flight had been on the runway for about 50 minutes when staff had informed the drone passengers.
"Everything was calm – frustrating rather than worrying," she said.
Michael, a flight passenger who did not want to give his last name, was less optimistic.
"I was afraid of having to camp here," he said.
He added that the problems here and in Gatwick showed "exactly how to close a country – this country – down," adding that the police had to "just shoot drones" as soon as it was view.
More than 140,000 passengers in Gatwick were hit during 36 hours of chaos between December 19 and 21.
About 1,000 flights were canceled in three days due to drone observations.