Islamabad, Pakistan (CNN) -- A car bomb ripped apart a university bus in Pakistan's troubled Balochistan province Monday, killing four people and injuring 40 others, police said.
The explosion took place in the outskirts of Quetta, the capital of the southwestern province, said Qazi Abdul Wahid, a senior Quetta police official.
The bomb was planted in a parked car and targeted a bus carrying university students, Wahid said. He said three students and a passer-by were killed in the blast.
Police said most of the injured were students.
It was not immediately clear who was responsible for the attack. But for years, Balochistan has been a target of militants who oppose the co-education of males and females.