PESHAWAR, Pakistan (AP) — A bomb exploded Saturday in a market in a northwestern tribal region that borders Afghanistan, killing at least 18 people and wounding at least 51. Shahid Khan, an assistant tribal administrator, said the explosion took place when the market was crowded with retailers buying fruits and vegetables from a wholesale shop. The Kurram tribal agency, where Parachinar is located, has a large Shiite Muslim population that has been attacked in the past by Sunni extremist militants who have hideouts there.