An Afghan policeman who is very likely a Taliban plant killed five embedded British mentors in the Nad Ali district in the turbulent Helmand province. From the BBC:

The British military has blamed the attack on a "rogue" Afghan policeman, who opened fire, injuring eight others, before fleeing the compound.

"While we are assembling evidence, the Taliban have claimed responsibility for this incident," the prime minister told MPs.

"It may be that the Taliban have used an Afghan police member or they have infiltrated the Afghan police force and that is what we've got to look at."

The dead soldiers had been mentoring Afghan police officers, working and living in a compound at a national police checkpoint in the Nad Ali district.

Later in the BBC report, a British officer smartly describes the impact these murders will have on British mentors:

A former commander of British forces in Afghanistan, Col Richard Kemp, said the shootings were a very worrying development. He said: "It will undermine trust, certainly in the short term, until we establish exactly what happened. And it wouldn't at all surprise me now if there aren't a lot of soldiers, British soldiers in Afghanistan, with their fingers very firmly on the trigger when they're around Afghan police and military."

I've spent a bit of time with US Military Training Teams and Police Training Teams, as well as with US units paired up with Iraqi Army and Sons of Iraq units from 2005-2008. Colonel Kemp has it right; the trust, not just with British mentors, will be degraded, and this trust is the most important factor in developing Afghan forces. The trainers can't train properly if their fingers are "very firmly on the trigger." Without this trust, conducting missions outside the wire becomes nearly impossible. The Taliban undoubtedly continue to seek such opportunities for attacks like this; the fact that such incidents are infrequent speaks volumes about their ability to do so.

While the Obama administration has yet to settle on the path forward in Afghanistan, one thing is certain: an expanded emphasis will be put on training Afghan forces. This will put more troops in direct contact with Afghan forces, and thus at greater risk of such attacks. NATO countries, fearful of the political fallout of increased casualties from such incidents, are likely to place greater emphasis on force protection for the trainers, which will build barriers between the Afghan forces and their foreign mentors.