By Andrew North
BBC News, Kabul
Morale in many police units in Afghanistan is low
The five British soldiers who were killed by a "rogue" Afghan policeman were mentoring the local police. Gordon Brown has described police training as an "essential element" of the strategy in Afghanistan. But what state is the Afghan force in and what are the challenges faced by Nato personnel?
The British soldiers would probably have been at their most relaxed, at the Afghan police post where they had been living and working for several weeks.
The danger was supposed to be outside the wire, from the Taliban. Then one of the policemen they had been training opened fire.
Why it happened is still being investigated. But it is perhaps significant that this incident involved the police.
Across the country, policemen are widely mistrusted because they so often demand bribes
While there has been progress building up a new Afghan army, the country's police force is still in very poor shape - despite huge financial investment over the past five years.
Boosting the size and effectiveness of Afghan security forces is a cornerstone of Nato strategy, in the hope they will eventually take over.
The US commander, Gen Stanley McChrystal, wants to increase the number of policemen from about 80,000 to 160,000 in just a few years.
But British troops working with the Afghan police in Helmand have frequently complained about their motivation, discipline and skills.
Across the country, policemen are widely mistrusted because they so often demand bribes.
Part of the problem is salaries. The basic wage is about £65 ($110) a month, well above the national average, but still very little to live on with rising prices.
Afghan soldiers, by contrast, are paid more and are also better trained.
Police recruits do just two months of training, while Afghan soldiers do six months before going on duty.
There are question marks too over the vetting of police recruits. Officially, they have to be approved by two serving members of the police.
But many recruits have been accepted simply on the say-so of one local district official.
Army recruits have to provide education certificates and references. Then there are further background checks into their relatives.
The police themselves complain they have been neglected.
They are often very badly equipped, compared with their army counterparts, despite facing similar risks from Taliban attacks.
It is common to find isolated checkpoints across Afghanistan with just a handful of policemen without body armour and with a few rusty weapons.
They are an easy target for the Taliban and are frequently overrun, with the police suffering much heavier casualties than foreign troops.
At least 1,000 police are reported to have been killed last year alone.
It is hardly surprising that morale in many police units is low.
Even with tighter checks, it is impossible to prevent incidents like the attack on British soldiers in Helmand. It has happened to the Americans, too.
There were similar incidents involving the deaths of two US personnel last month and two more in 2008.
British, US and other foreign troops are with Afghan police and troops every day.
And General McChrystal's plan will mean them spending even more time with Afghan security forces. That, in the end, is Nato's exit strategy.