What happened on night of deadly Afghanistan helicopter crash?

Late last Friday night, special forces troops from the NATO-led coalition launched an operation to capture a Taliban leader in an inaccessible valley southwest of Kabul.

A few hours later 38 troops -- 30 of them Americans -- lay dead in a transport helicopter destroyed in the worst single incident suffered by foreign forces in 10 years of war in Afghanistan.

Little, if any, information was available soon after the crash, mainly because "a cone of silence had been ordered from the top," one senior military official said.

Reuters has been able to reconstruct a clearer picture of the circumstances of the crash after interviews with officials from the International Security Assistance Force (ISAF) in Afghanistan and the U.S. military.

Unless identified, all spoke on condition of anonymity because investigations are still being carried out.

The disaster unfolded after an ISAF Special Operations Command (SOC) team that included at least some U.S. Rangers began a raid in the Tangi valley in central Maidan Wardak province under darkness late Friday.

Typically carried out in conjunction with Afghan soldiers, "night raids" anger ordinary Afghans who complain they do not respect their privacy or Islamic culture. However, they are one of the most successful tactics used by foreign troops hunting insurgents who hide among Afghan civilians.

Only 80 km (50 miles) southwest of Kabul, the valley is ringed by rugged mountains. Rocky outcrops are dotted around the surprisingly lush valley, making it easy for insurgents to hide, monitor troop movements and control access to the valley.

There are a couple of sparse settlements, with outlying compounds near narrow waterways that snake through the valley.

Despite -- or because of -- the valley's proximity to the capital, it has long been a hub for insurgents. The Taliban, the al Qaeda-linked Haqqani network and others are active in one of central Afghanistan's most dangerous areas.

An earlier SOC mission had killed Taliban cell leader Din Mohammad and Friday's mission targeted Mullah Mohibullah, who led a network of 12 fighters in the Tangi valley.

UNDER FIRE

"It was a capture operation, a standard night operation," one senior ISAF official said.

As the SOC team moved through the valley, they soon saw insurgents armed with AK-47 assault rifles and rocket-propelled grenade launchers, the most common weapons used by insurgent foot soldiers for their ease of use, maintenance and resupply.

Military officials in Kabul say six members of Mohibullah's network had already been killed as the SOC team engaged them. The Taliban said Saturday that eight of its fighters had been killed during the exchange.

That firefight is believed to have taken place around one of the compounds in the valley.

"The ground force was assaulting the objective and were in contact with the insurgents," another military official told Reuters, adding that a small number of Taliban fighters soon broke away from the main group.

At that point, very early Saturday, the SOC ground team called in what ISAF describes as an "Immediate Reaction Force" (IRF), a standby unit.

An IRF is different from a Quick Reaction Force, emergency units that have been deployed in response to "spectacular" attacks by insurgents in Kabul and elsewhere.

Despite widespread speculation to the contrary, that means the extra force called in to assist the ground team was not sent on a "rescue" mission. Neither was it caught in any kind of elaborate Taliban trap.

"A group started breaking away and fleeing," the second military official said. "That's when they called in the IRF, to come in and get those guys."

Military officials have said that, under such circumstances, it is not unusual for insurgent leaders to break off from an engagement and leave behind "low-level fighters."

"That's when the helicopter coming in got hit," one said.

Several military and diplomatic officials have said it appeared the devastating death toll -- 30 U.S. troops, seven Afghan commandos and an Afghan interpreter -- was the result of nothing more than a lucky shot.

"It just has to be the right angle, the right shot," one said.

ISAF says insurgents had been using RPGs during the initial engagement and that the helicopter had been fired on, but still refers to the incident as a crash.

"While it has not been determined if enemy fire was the sole reason for the helicopter crash, it did take fire from several insurgent locations on its approach," ISAF said.

While it is not unusual for rocket-propelled grenades -- normally an anti-tank weapon -- to hit helicopters, it is extremely rare for them to actually bring one down.

Coalition officials have effectively ruled out that the helicopter was brought down by anything more sophisticated than an RPG launcher. That dispels fears the Taliban had suddenly acquired more sophisticated weapons such as the Stinger missiles used to such devastating effect by mujahideen fighters against Soviet aircraft during the occupation of the 1980s.

RPGs have an effective range of about 300 metres, although officials in Kabul say the shot that downed the Chinook would have to have been fired well within 100 metres of its target.

"The shot could have come from a low angle, or even from above the helicopter," one military official said.

What is not known for sure now is whether the helicopter caught fire or exploded, or whether it fell from any considerable height. Officials acknowledge that the destruction was devastating, something supported by the fact it took about four days to gather all of the wreckage and remains.

"Whether it was a fire or if it exploded, it was catastrophic," another military official said.

"It's a bloody big target, and a slow-moving target."

As soon as the helicopter crashed, the SOC ground troops rushed to the secure the scene. It now appears Mohibullah and his remaining fighters then escaped.

Mohibullah and the unidentified man who fired the shot at the helicopter attempted to flee the country, ISAF said, most likely to Pakistan, but were tracked to a wooded area in a nearby district. Both were killed by an air strike Tuesday, ISAF said.

Questions have been asked in the United States why the second unit -- which consisted of 25 members of the Navy's Seal Team 6 -- was traveling in a U.S. Army CH-47 instead of a more sophisticated MH-47 more commonly used by special forces.

Part of that explanation might lie in the fact it was not the primary unit used in the raid and was only on standby.