BBC Kabul correspondent
The region of southern Afghanistan where three British soldiers have been killed by US friendly fire is a war zone where the close-quarter fighting is intense and the risks are high.
Fighting in Afghanistan is intense and dangerous
The Royal Anglian troops were picking their way through the thick mud-walled compounds and irrigation ditches of Kajaki on another early evening patrol.
It was not unusual for them to come under fire - it regularly happens when British troops push from their base out into the no-man's land a few kilometres from the dam they are protecting.
They scrambled to their positions and returned fire - they were being attacked from a number of different directions.
As usual, the decision was made to call for air support and two American F-15 fighters were soon buzzing overhead.
What happened next will be at the heart of the investigation.
Air power vital
A bomb was dropped, but rather than hitting the Taleban positions it struck a group of British soldiers - three were killed and two others injured.
"There are a handful of different reasons why this tragic incident may have happened," said Lt Col Charlie Mayo, spokesman for the Helmand Task Force.
"We are investigating the circumstances, but we are not in a position at the moment, and I don't think we will be for some time, to find out exactly why this happened."
Officers insist they are making military progress
Air power is a vital part of the Nato arsenal and it gives the international forces a major advantage over the Taleban in terrain which makes fighting a guerrilla war even more difficult.
But even with modern guidance systems, mistakes can be made and civilians - as well as troops - have been killed over the past few months.
In December last year, Royal Marine Jonathan Wigley, 21, from Leicestershire, died from injuries sustained in an American bombing raid.
He and his unit were ambushed in Garmsir, another town in Helmand. They were outflanked and surrounded on three sides and the air power was needed to allow them to withdraw - but the bomb was dropped too close to British positions.
Canadian troops have also been killed or injured by US aircraft.
The intense nature of the close-quarter fighting does increase the risk as bombs are being dropped very close to Allied forces.
The commander of British forces in Helmand province, Brig John Lorimer, sent his condolences to the families of those killed.
"It's a terrible tragedy and we have been working with the US air force a great deal out here and many, many times, the bombs have saved the lives of hundreds of British troops," he said.
Officers insist they are making military progress, pushing Taleban fighters further back from the main towns and killing senior commanders.
But the mission is to try to win Afghan villagers over for the government and efforts to bring development, justice and better governance are proving challenging, as is mentoring the new Afghan National Army and police to take more of a responsibility for national security.