A suicide bomber rammed a vehicle packed with explosives into a police station in Pakistan's restive Swat valley on Thursday, killing four people and destroying the building, police said. The attack was the latest in a string of bombings on security forces in troubled northwestern Pakistan, where the government is waging a military campaign against militants with links to Al-Qaeda and the Taliban. "It was a huge blast. Three security force personnel and one policeman were killed. The police station was destroyed," Mingora city police chief Dilawar Khan Bangash said. He said the coordinated attack had begun soon after midnight, with militants and security forces trading fierce gunfire before the suicide bomber drove a 14-seat bus into the back of the police station's grounds. The vehicle exploded as it came under fire, damaging scores of shops, a hotel, a school and many houses in Mingora, the main town in the Swat valley. Most of the 30 people injured were security forces and police, though two civilian bystanders were also hurt, he said. The building was wrecked by Thursday's attack, with its heavy concrete floors blown apart and its walls reduced to rumble.