Masked gunmen hurled grenades and opened fire on a government pickup truck in northwestern Pakistan on Tuesday, killing at least four people, including a senior intelligence agent and two of his aides, officials said. The attack occurred on a road in Bajur, a Pakistani tribal region bordering Afghanistan, said Mowaz Khan, a security official in Khar, the main town of Bajur. In the attack on a road near Khar, two men wearing masks and riding a motorcycle threw two grenades at the official vehicle and then opened fire, killing the intelligence official and three people at the scene, The Associated Press quoted Khan as saying. Khan and an intelligence official in Peshawar, the capital of North West Frontier Province, said four people were killed in the assault. The driver of the pickup was wounded, Khan said. Two passers-by in another car were also injured in the shooting, the Peshawar-based intelligence officer said. The attackers fled.