At least four people were killed, including four police officers, in a suicide car bomb attack in the southwestern Pakistani city of Quetta on Friday, officials said.

The explosion near the police chief’s office in Quetta, the capital of Baluchistan province, was powerful enough that it was heard across the city, shattering windows of nearby buildings, said police spokesman Shahzada Farhat.

Nineteen people were injured in the blast, said Wasim Baig, a spokesman for the Civil Hospital in Quetta. Some people remained in in critical condition.

Abdul Razzaq Cheema, director general of police in Baluchistan province, told Reuters it appeared the bomber had detonated a car packed with explosives.

Four of those killed were police officers, while nine security officials were among the wounded, said Fareed Sumalan, a doctor at the Civil Hospital.

Jamaat ur Ahrar, a splinter group of the Pakistani Taliban, claimed responsibility for the attack in a message sent to Reuters by spokesman Asad Mansur.

Baluchistan government spokesman Anwar ul Haq Kakar said the blast happened near the office of the inspector general of police when officers stopped the car to search it near a checkpoint.

“It’s possible the IG office was the target, or the assailants were trying to enter the (army) cantonment which is close by,” he said.

Television footage showed emergency services rushing to the scene, which was covered in debris, while security officials cordoned off the area.

The normally busy intersection – which lies between a girls’ school, a girls’ college, the police office and the entrance to the army cantonment – was relatively quiet because Friday was a public holiday ahead of the Eid celebrations marking the end of the holy Islamic month of Ramadan.

Quetta is about 100 km (60 miles) east of the border with Afghanistan and is the capital of Baluchistan, which has been plagued by militant violence. The province is the base of a separatist movement as well as home to the Taliban and other militant groups.

It is also at the center of Chinese-backed “Belt and Road” development projects worth $57 billion that first focused on Chinese firms building roads and power stations but is now expanding to include setting up industries.

Militants from the ISIS terrorist group abducted and killed two Chinese nationals in Quetta last month.

That attack prompted Pakistan to boost security around Chinese nationals and other foreigners in the province, which is already one of the most militarized regions in the country.