[img]http://d.yimg.com/a/p/ap/20100828/capt.c6d21b675a31425c9c737d3f76c74862-c6d21b675a31425c9c737d3f76c74862-0.jpg?x=130&y=80&q=85&sig=iO_6owegHLpc9GsIwbXduw--[/img]AP - A heavy exchange of fire erupted Saturday at an office of security forces in northwest Pakistan, but the motive behind the attack was not immediately clear, police said.