The gun used in the fatal shooting of a woman in San Francisco belonged to a federal agent, sources confirmed Tuesday. Francisco Lopez Sanchez had earlier pleaded not guilty to a murder charge in the killing of Kate Steinle, who was shot once in the back last week while walking with her father along the Pier 14 tourist spot. Sanchez had said he found the gun wrapped in a T-shirt on the ground near a bench and that it accidentally went off when he picked it up. The Mexican national, who has been deported five times, also said he had been taking sleeping pills. No additional details about the weapon were available, but one source told KGO-TV the gun may have been stolen from an agent’s car. The case has reignited a national immigration debate and the policies of sanctuary cities such as San Francisco, which offers certain protections to undocumented immigrants.