US Defense Secretary Leon Panetta assured Afghan President Hamid Karzai in a phone call Sunday that a \"full investigation\" was under way after a US soldier allegedly killed 16 civilians in Afghanistan. \"A suspect is in custody, and I gave President Karzai my assurances that we will bring those responsible to justice,\" Panetta said in a statement, adding that he is \"shocked and saddened that a US service member... clearly acting outside his chain of command\" has been linked to the incident.