While investigators declined to speculate on a motive for the killing of Sabeen Mahmud, friends and colleagues immediately described her death as a targeted assassination in Pakistan, a country with a nascent democracy where the military and intelligence services still hold tremendous sway. The gunmen shot both Mahmud and her mother, Mehnaz Mahmud, as they stopped at a traffic light Friday night in an upscale Karachi neighborhood, senior police officer Zafar Iqbal said.