Detectives have appealed for information after a drone apparently hit a British Airways flight as it came in to land at Heathrow.
The pilot of flight BA727 from Geneva in Switzerland reported being hit as the Airbus A320 approached the west London airport on Sunday afternoon with 132 passengers and five crew on board.
Scotland Yard believes the incident occurred about 1,700ft in the sky above Richmond, south-west London.
Police said officers have searched a "wide area" for suspects or debris but nothing has been found.
Anyone who discovers drone parts in the Richmond area, or who was in Richmond Park or surrounding open spaces between noon and 1pm on Sunday and has relevant information, is asked to contact aviation police.
No arrests have been made over the incident.
The Air Accidents Investigation Branch has also begun an investigation.
Justin Pringle, chief technology officer at Newcastle-based firm Drone Operations, told the Press Association there "isn't any chance of catching the pilot because drones do not have to be registered".
He described the collision as "an untraceable event".
Transport minister Robert Goodwill said the Government is looking at the possibility of introducing a registration scheme in the UK, similar to ones already in place in Ireland and the US.
Pringle warned that anyone can walk into a shop, hand over £1,000 and walk out with a drone.
"At no point have you had a conversation about safety or the rules and regulations attached to that vehicle," he added.
The collision is the latest and most serious in a string of incidents involving drones at Heathrow, with several near misses between flights and unmanned aircraft in the last year.
It raises the issue of regulation and control of drones, especially in sensitive areas like airports.
BA said the aircraft landed safely and was examined by engineers before being cleared to take off on its next flight.
Chief Superintendent Martin Hendy, head of the Met's aviation policing command, said: "Thankfully the aircraft landed safely but the incident highlights the very real dangers of reckless, negligent and sometimes malicious use of drones.
"We continue to work with the Civil Aviation Authority and other partners to tackle this issue and ensure that enthusiasts who fly drones understand the dangers and the law.
"One of the challenges is to ensure people realise what is legitimate and what is illegal. The message is do not fly them anywhere near airports or flight paths, or over crowded places such as football and other stadiums. The potential is there for a major incident."
Steve Landells, flight safety specialist at the British Airline Pilots Association (Balpa), called for greater enforcement and awareness of rules that govern drone flights.
He said: "Frankly it was only a matter of time before we had a drone strike given the huge numbers being flown around by amateurs who don't understand the risks and the rules."
The Civil Aviation Authority advises that drone operators must be able to see the craft at all times and must not fly above 400ft.
Drones fitted with cameras must not be flown within 50 metres of people, vehicles or buildings, or over congested areas or large gatherings such as concerts and sports events.
A report in March by the UK Airprox Board (UKAB) found there were 23 near misses between drones and aircraft in the six months between April and October last year, including two at Heathrow.
A CAA spokesman said it was "totally unacceptable" to fly drones close to airports and anyone flouting the rules can face severe penalties including imprisonment.