Azam Tariq, a spokesman for the Tehrik-e-Taleban Pakistan (TTP), claimed responsibility for the attack, which he said was in retaliation for US air strikes in the country’s northwestern tribal areas.

“It’s a reaction to the drone strikes and such attacks will continue until drone strikes are stopped,” he said.

The explosion rocked a high-security area that is home to the US Consulate, two luxury hotels and the offices of government leaders, showing the reach of militants in the city despite efforts to crack down on them.

Gunman first opened fired on the office of the Crime Investigation Department before detonating a massive car bomb, said Sindh Home Minister Zulfiqar Mirza. The building has a detention facility that was believed to be holding criminals, and possibly militants.

The CID takes the lead in hunting down terrorists in Karachi.

Earlier this week, the agency arrested six members of the militant Lashkar-e-Jhangvi group. The suspects were presented before a court earlier Thursday.

Broken glass littered the area for up to two km from the bomb site, where rescue workers installed search lights in the dark as they dug frantically through the rubble for survivors, a reporter said.

Dozens of vehicles were destroyed or damaged in the attack, the aftermath of which saw ambulances screeching through the streets, ferrying casualties to hospital.

“The building has been completely destroyed. I can see a crater of 15 feet,” police official Tariq Razzaq Dharejo said.

“We heard different kinds of firing for several minutes and then a deafening explosion,” said Ali Hussain, who was covered in dust. “The roof of our house collapsed.”