A huge explosion outside a police building in the Pakistani city of Lahore has killed at least 23 people and injured about 250. Officials said gunmen opened fire from a car which drove up to the building, near the provincial headquarters of Pakistan's main intelligence agency, the ISI. After police returned fire, the car exploded, damaging buildings over a wide radius. Rescue workers are digging through the rubble for survivors. While no one has claimed responsibility for the attack, Taliban militants have threatened retaliation for the government's current offensive in the northwest of the country. For more, we turn to Rob Watson, defense correspondent for the BBC World Service, who is following the story.