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Bat-winged, high-flying and hard to detect, America's RQ-170 Sentinel plane is the perfect stealth drone for peering into another country's secret sites without being caught.

One was used in May to feed back live footage of the US Navy Seal raid on Osama Bin Laden's compound in Pakistan.

So probably not the sort of hardware the CIA would ever like to fall into the hands of the Iranian Revolutionary Guards Corps? Oops.

On 4 December, around 140 miles inside Iran from its border with Afghanistan, that is exactly what has happened.

On Thursday afternoon, Iran displayed its captured trophy on TV, apparently perfectly intact and, according to the Iranian media, Russian and Chinese military intelligence officials are taking a keen interest in it.

Opinion is divided on how this hi-tech intelligence-gathering drone fell into "the wrong hands" and, indeed, what it was doing inside Iran.

Built by Lockheed Martin, unveiled at Kandahar Airbase in 2009 and capable of flying at an altitude of up to 50,000ft (15.2km), this Unmanned Aerial Vehicle (UAV) carries no missiles, unlike the larger, lethal drones, the Predator and the Reaper, that also fly from bases in Afghanistan.

The US says simply that its Sentinel had a malfunction, but the plane is supposed to have a failsafe back-up system that automatically steers it back to base if contact is lost with its controller.

Sophisticated sensors

The base in this case is Shindand in western Afghanistan, a former Soviet airbase from where US-operated drones are used to monitor the movements of Taliban insurgents and smugglers along the long border with Iran.

But speculation is rife that this particular aircraft was flying deep inside Iran to gather intelligence and real-time video footage of Iran's nuclear sites.

It was carrying an array of sophisticated sensors that will be of great interest to Iran and other countries.

If, as was originally thought, the Sentinel had been shot down then there would have been little to put on display but a pile of twisted wreckage.

Instead, what was on show on Iranian TV was an immaculate gleaming white drone that looked straight off the production line.

Which tends to back up the claim by Iran that its forces brought down the drone through electronic warfare, in other words that it electronically hijacked the plane and steered it to the ground.

On Thursday, the Commander of Iran's Islamic Revolutionary Guards Corps (IRGC) Aerospace Force Brig-Gen Amir Ali Hajizadeh said "through precise electronic monitoring it was known that this plane had the objective of penetrating the country's skies for espionage purposes.

"After entering the country's eastern space the plane was caught in an electronic ambush by the armed forces and it was brought down on the land with minimum damage."

This affair is both a political embarrassment and an intelligence setback for Washington.

It is also unlikely to help those countries like Britain that are trying to obstruct and delay what they suspect is an Iranian nuclear weapons programme - a programme Tehran denies.

Iran has now formally complained about the US intrusion into its airspace and asked for compensation.

In the CIA Directorate of Intelligence at Langley, Virginia, eyes will be rolling skywards as analysts work out the long-term damage to US intelligence.

Not only must they accept that some of their most successful and useful surveillance technology is now in the hands of the very people they were using it on, they will also have to think very carefully before sending anything else into Iranian airspace.

Above all, they must be asking: does Iran really have the capacity to intercept transmissions between our stealth drones and our controllers on the ground?