A bomb and shooting attack blamed on Tamil separatists ripped through a packed civilian bus Wednesday, killing 26 people in southeastern Sri Lanka as the government officially withdrew from a tattered cease-fire with the rebels. Military spokesman Brig. Udaya Nanayakkara said it was clear the rebels were behind the assault, the latest in a string of attacks in government-held territory in recent months. "There are no other groups operating in the area," he said. Spokesmen for the rebels could not immediately be reached for comment. But the group, listed as a terror organization by the US and European Union, routinely denies responsibility for such attacks. If the Tamil Tigers are responsible, it highlights their increasing determination to hit targets in the generally peaceful south as the military presses ahead with an offensive against rebel-held territory in the north.