KARACHI (Reuters) - Staring blankly at what used to be his office, the plain-clothed police investigator recalled how he had worked long hours there trying to track down some of Pakistan’s most dangerous militants.

The office was flattened in a suspected Pakistani Taliban suicide car bombing on Thursday at the compound of the Crime Investigation Department (CID), which focuses on apprehending militants who are bent on toppling Pakistan’s U.S.-backed government.

“I arrested members of the Lashkar-e-Jhangvi (LeJ),” said the investigator, who looked lost, his AK-47 assault rifle slung over his shoulder.

LeJ is one of Pakistan’s most violent anti-Shi’ite groups and part of an al Qaeda-linked nexus of militants. They include the Pakistani Taliban, which claimed responsibility for the attack, which killed 15 people and wounded 100, and threatened more bloodshed.

The explosion left a crater about 40 feet across and 12 feet (four meters) deep in front of the gutted building in Pakistan’s financial capital and biggest city, where senior militants were held and interrogated.

The police investigator pointed out one of the rooms which held militants. It was reduced to broken concrete slabs and twisted metal.

Parts of nearby buildings collapsed in the attack, which took place meters from the provincial chief minister’s house in a central district known as the “red zone” because of its high security status. The U.S. Consulate and five-star hotels are nearby.

Slideshow ( 3 images )

A security guard knelt in the rubble and sifted through a pile of bullet casings. Police said the militants first opened fire, as they sometimes do during such attacks.

Official documents were strewn across the rubble.

Security officers and rescue workers tried to steady themselves over shaky chunks of cement as they searched furiously for possible survivors. A pair of sandals raised hope, but to no avail.

A few feet away a group of security officers grunted as they pushed away a huge block of cement. “Allah-u-Akbar” (“God is Great”), they shouted, hopeful they had found a survivor.

But a few minutes later the body of a paramilitary soldier, his face covered in dust, was carried away on a stretcher.