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LONDON, England (CNN) -- What caused a Boeing 777, one of the world's most reliable jet planes, to drop from the skies over London Thursday and come within seconds of disaster?

An emergency vehicle is shown with one of the plane's wheels, torn from the undercarriage, in the foreground.

The Air Accidents Investigation Branch of the UK's Department for Transport is now investigating the incident. A team from the U.S. National Transportation Safety Board is also heading to London, accompanied by representatives from Boeing and the Federal Aviation Administration.

Investigators will quiz all involved with the flight, both on the ground and in the air, as well as check the flight data and cockpit recorders.

Debate in aviation as to what has caused one of the world's most reliable planes to crash land at the world's busiest international airport has now gathered pace, with several theories now emerging.

Was it down to terrorism?
Heathrow Airport has long been seen as potential target for terrorist activity. In August 2006, police claimed they had foiled a plot to blow up aircraft flying from the United Kingdom to the United States.

But shortly after Thursday's incident a spokeswoman for Scotland Yard, the UK police headquarters said: "There is nothing to suggest it is terror-related."

No terrorist group has tried to claim responsibility for yesterday's events. And there have been no reports, either from the ground or the air, of any explosion. No expert has come forward to suggest this as a possible cause.

What about bad weather?
Weather in the region at the time of the crash land was not extreme but there have been anecdotal reports of adverse conditions. CNN digital producer Barry Neild, who was in Sheen, just east of Heathrow, at the time of the incident, found himself caught in a brief hailstorm and also witnessed lightning for a short time.

Robert Cullemore, of London-based aviation consultancy Aviation Economics, said a pilot from a rival airline told him that officials were seriously considering weather as a major factor.

Cullemore said they were examining the possibility that a sudden gust of wind -- known as wind shear -- may have affected the flight.

"It can happen anytime anywhere and if it happens you just hope there is no airplane nearby," said Cullemore.

Aviation Web site flightglobal.com reported that general windspeed in the area at the time of the incident was 16kt (30km/h) "with a warning that the wind might vary temporarily to 240° at 20kt (37km/h), gusting to 32kt (60 km/h)."

The site also reported the temperature at 11°C, visibility at 10km (6.2 miles) and broken cloud at 1,400ft and 20, 000ft and visibility at 10km (6.2 miles).

Mike Giles, a retired CX B747-400 captain who has 18,500 flying hours and piloted the 777 in training, told CNN that the first thing to go through any pilot's mind when they hear of an incident such as Thursday's was what might have caused it.

He said that an incidence of wind shear or a microburst would push the aeroplane down but then it would lose speed and possibly would not have crash landed as fast as it did

Could it have been pilot error?
The central witness to Thursday's events will be pilot Peter Burkill, 44, who has been praised for how he handled the incident.

British Airways Chief Executive Officer Willie Walsh said: ``I would like to pay tribute to the 16 crew of the BA038 led by Captain Peter Burkill,'' said Walsh. British Airways has employed Burkill, who it has called one of its most experienced pilots, for near on 20 years.

David Kaminski-Morrow, of Web site Air Transport Intelligence, said: "Landings and take-offs are the two key parts of a flight, with take-offs testing the plane's systems and landings examining the pilots' abilities," in comments carried by the UK's Press Association.

Said Giles: "The fact that they all walked away from it means to me that the crew were fully conversant as to what was going on. It just happened so late in the piece that they just did not have the time to call anybody."

Are they looking at mechanical and/or electrical failure?
An investigator who has been briefed on the incident told CNN that the plane's captain "is claiming there wasn't power when he needed it."

Passengers have also said that what happened occurred quickly, with no warning to adopt an emergency brace position. Several reports also suggest that the pilot had no time to radio the tower at Heathrow.

Witnesses on the ground have spoken of the engines making a strange rumbling noise as the plane approached.

But the Boeing 777 is considered a staple of major airlines' long-haul fleets and has never been involved in a fatal accident since entering service in June 1995.

Kaminski-Morrow added: "Take-off and landing incidents tend not to be the result of technical problems."

And Giles said that jet engines can only stop through lack of air or lack of fuel. "I can't think of an aeroplane that has lost two engines in seconds before. It just doesn't happen." Another cause, he suggested, may have been down to an aircraft configuration problem such as the landing flaps.

Maybe it was a bird strike?
Heathrow Airport does have reservoirs nearby which are often havens for migratory birds. Several aviation web sites have claimed that a flight leaving Heathrow for Beirut last autumn had to turn back because of an alleged bird strike.

The Web site for Aviation Safety & Security Digest noted that an A320 had its "radome crushed and crumpled by a nasty bird strike" on October 22, adding "Not caused by a flock of Canada geese, actually identified as a migrating song thrush (hit at 6000 ft at 250kts indicated airspeed)."

However, the radome of a plane -- the protective housing for the radar antennae - is much smaller in size than the engines.

Giles told CNN: "There is no way in this world that birds would stop two engines within seconds of each other.

"You may see a little change (in engine performance) but most of them will get spat out the other end. The chances of it stopping the engine dead, and a flock of birds stopping both engines at the same time, I think are a million to one."

Whatever the cause of Thursday's incident, one fact remains: the incident could have been much worse. As Kaminski-Morrow told PA: "Investigators ought to be able to get to the bottom of this as they can at least talk to everyone involved as this was a non-fatal accident with everyone pretty much being able to walk away."

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