Islamabad, May 21 (Reporter) At least four students were killed and over 40 others injured when a bomb went off near a school bus in Pakistan's southwest Balochistan province on Wednesday, an official said. The explosion happened when a school bus carrying children and staff members passed by a vehicle loaded with explosives and parked along the roadside near the zero point area of Khuzdar district of the province, the police and government officials said. He said that the condition of several blast victims is critical, adding that a state of emergency has been imposed and all medical staff of hospitals in the district have been called in. Eyewitnesses in the area told Pakistani media that the bus was completely destroyed after the powerful explosion. Following the blast, the police cordoned off the area and launched a search operation in the surroundings to arrest the culprits, Dawn reported. No group has claimed responsibility for the attack yet...////...