I have always believed the term "Afghan police force" was a misleading one. The men who make up the force are not trained police officers in any sense that we would recognise. They are tribal militiamen who are wearing a uniform. Very few have been through a police academy and most are entirely untrained. Two guys are grabbed from a village, stuck in uniform and told: "You are now police." That's how it happens.

I served in Afghanistan twice with the British army, mentoring Afghan soldiers and police officers throughout Helmand in 2006/7 and again, with the Royal Irish Rangers, last year. And whether people will admit this openly or not, it is a fact that the Afghan police have been infiltrated at every level by the insurgency. They are not employed by governments, they are paid by governments – and if the Taliban come along and pay them more, their loyalties can be very fluid. The corruption that runs so deeply through Afghan culture is what drives them, and their motivations are less ideological than financial. So it is entirely possible that this incredibly shocking incident was because this young man had been influenced by the insurgency.

But there are other reasons why this could have happened. For one thing, the level of drug-taking within the Afghan police is incredibly high – it's thought that around 80% have an active heroin addiction – and it's possible that the gunman was simply high on drugs. That happens all over Helmand.

It's also very possible that the incident could have arisen from a grievance. In general the British troops and Afghan police get on pretty well – we work hard at that, and there is a certain amount of banter between us, sharing stories about our families and our hopes for the future – but you know what soldiers are like. Someone could have said or done something and a situation of conflict arose.

What is termed "mentoring" means literally living every minute of the day alongside the Afghans, trying to help them achieve their mission. The day is spent talking to them, explaining how to keep their camp clean, their weapons clean, how to organise their patrols. It's about walking alongside them, helping them to achieve the mission of that particular patrol, helping out with logistics. After patrol you return to camp and you sit down together for lunch, eating cross-legged and with your right hand, as they do.You're there to fight alongside them until they are able to do it themselves.

We have to be seen to be standing shoulder to shoulder with these people in order to mentor them, and we can sometimes let our guard drop. Procedurally, I think British troops will now have to ensure that someone is being vigilant at all times, but it's important to remember that this kind of incident is very rare.

Inevitably this will make the British troops a little nervous but these are professional men. I would have confidence that the units that have suffered losses will mourn the passing of their comrades, then gather themselves and get straight back to work.

• Captain Doug Beattie was awarded the Military Cross for his part in the battle of Garmsir in Helmand in 2006. He retired from the army this year. His book, Task Force Helmand, was published last month