British and Afghan commanders were last night undertaking an urgent investigation into the killing of five British soldiers amid concerns that the Taliban may have infiltrated the police force in Helmand. The troops were killed by a local police officer who opened fire as they sat drinking tea and fled.The deaths prompted deep soul searching in Whitehall because Gordon Brown has put the training by the British army of a rapidly expanded Afghan security force at the heart of his exit strategy from Afghanistan.Faced by Labour backbench calls for a phased withdrawal, Brown said the work of the troops must continue. "We must not allow ourselves to give up what the Afghan Taliban fear most: that we will have a ...