The half-brother of Afghan President Hamid Karzai has been assassinated in Kandahar.Ahmad Wali Karzai, a leading power broker in the country’s south, was shot dead at his home in a blow to Nato's battle against the Taliban in the area.He was shot twice by his long-time head of security Sardar Mohammed, who was himself killed almost immediately.The Taliban said they carried out the attack, calling it one of their top achievements in ten years of war.Lyse Doucet reports.