The US forces who raided the safehouse of Osama Bin Laden appear to have caught him completely by surprise - and to have avoided detection by Pakistani radar. How did they do it?

One answer, experts believe, is that the special operations team used previously unseen stealth helicopters.

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1. Silver finish makes it harder to detect on radar and by infrared sensors

2. Shape of tailboom has been altered and possibly enlarged to evade radar

3. Pan-like cover or hubcap over the rear rotor head conceals exposed machinery which is more easily picked up on radar

4. Extra blades on tail rotor reduce noise and lessen typical chopper sound

Rest of aircraft: There is speculation that the main rotor could also have had extra blades, retractable landing gear and a cover over the main rotor head

Source: Aviation Week

The evidence for this comes from images of the wreckage of one of the helicopters, which departing Seals destroyed after it crash landed in the compound.

The tail of the top secret aircraft survived, however, providing a treasure chest of clues for aviation experts.

After some detective work, these experts have concluded it was a UH-60 Blackhawk, heavily modified to make it quieter and less visible to radar.

They are confident the raid marks the first time that a stealth helicopter has been used operationally.

It wouldn't be the first of its kind in existence, however. Sikorsky Aircraft built a number of prototype stealth helicopters, known as the RAH-66 Comanche, for the US Army. The programme was cancelled in 2004, due to escalating costs, before the helicopter become operational.

The RAH-66 Comanche had some stealth technologies

"What's new here is this was operational use," says Bill Sweetman, editor of Aviation Week. "We really haven't seen stealth helicopters used in this way before.

"The bottom line is about increasing the element of surprise. The less warning that the target has the better."

In this kind of situation an escape route for the aircraft might be needed, he adds, in which case time is of the essence.

To make a helicopter stealthy, you have to get rid of certain shapes and areas that are easily picked up on radar, says Tony Osborne, deputy editor of Rotorhub, a UK-based helicopter magazine.

"You have to cover key parts so that the radar waves bounce in different directions or get absorbed...

"The tail rotor gearbox is covered. I've never seen that before in a helicopter. We know things are being played with all the time, but it is impressive to see it put into action."

The tail fin is completely smooth and appears coated in a pearlescent material that looks silver in some lights, and black in others, says Mr Osborne.

"I've only ever seen that on stealth aeroplanes, and it would probably absorb radar waves. Even the rivets are covered - radars are very sensitive and small rivets could give it away.

"The tail boom remains suggest the landing gear was retractable - again, usually it could be detected by radar, so retracting it would help avoid radar detection.

t looks like the tail rotor has five or six blades. This would mean the rotor could have a slower rotation, which would mean less noise. Noise is caused by the blade tips spinning at high speed, hitting the air."

Slipping under the radar can also be possible without stealth technology. Most of Pakistan's radars are on the ground, and therefore angled in such a way that makes low-flying aircraft difficult to detect, Mr Osborne says.

A Pakistani intelligence official who wished to remain anonymous told the BBC's Syed Shoaib Hasan that the helicopters were not picked up on radar and were only detected whenseen entering the country from Afghanistan.

He said there were four helicopters, coming in very low.

There has been speculation that there was one more stealth helicopter, identical to the one that crashed, and that these were used as pathfinders, backed up by two larger Chinooks.