In Karachi's largest mortuary, the body of an infant not more than nine months old sits waiting to be claimed.

He was the youngest victim of a suicide attack at a shrine in southwest Pakistan Saturday that killed 52 people. Mortuary worker Latafatullah Hassan said he fears the infant's parents were also among the dead.

The mortuary is cold. The child's body is uncovered. He looks as if he could be sleeping, with no visible wounds from the suicide bombing of a crowded Sufi shrine in the midst of a crowded religious celebration.

The Islamic State group's local affiliate, calling itself the Khorasan Province, claimed responsibility — saying that a suicide bomber had attacked "Shiites" at the Shah Noorani shrine in the remote Khuzdar district of Baluchistan province. The bomber struck amid diverse crowds full of women and children during the traditional Sufi dhammal dance. The shrine is frequented by both Pakistan's majority Sunni and minority Shiite Muslims.

Gulam Rasool arrived frantically at the mortuary, waving the identity card of his cousin — an elderly looking man with a long white beard. "I've been everywhere, to every hospital but I can't find him. They told me to come here."

An ambulance with a body inside screamed to the gate. Rasool shoved past people to reach the vehicle. But it wasn't his cousin.

Inside the ambulance was the corpse of 16-year-old Bilal, accompanied by his father, Sabir. The older man's brown shalwar kameez was soaked with sweat and tears covered his cheeks. Sabir, who didn't give his family name, said his only son had been on his first-ever visit to the shrine with a group of friends. Now he was bringing his body to be washed and wrapped in a white shroud in keeping with Islamic burial custom.

Senior police official Azad Khan said the area around the shrine is a known hotbed for sectarian militants. It is the home turf of Shafique Mengal, who was once a leader in the virulently anti-Shiite Muslim group, Lashkar-e-Jhangvi. Two years ago Mengal declared allegiance to the Islamic State group.

Khan said that Mengal and his followers have found safe havens across the border in Afghanistan. He also said that both local and foreign militants, including Uzbek nationals, have begun carrying out attacks on behalf of IS.

In Karachi, paramilitary rangers and police have raided dozens of madrassas, or Islamic religious schools, in recent days in search of militants aligned with militant sectarian groups. Meanwhile the frequent Sunni militant attacks on Shiites, who make up about 25 percent of Pakistan's 180 million people, have resulted in retaliatory attacks on Sunnis from a Shiite militant group known as the TNFJ.

Dr. Abdiul Qadir Siddiqui, chief of Karachi's trauma center, said the majority of the injuries were caused by ball-bearings that peppered the bodies of worshippers when the suicide bomber detonated his explosive device. Authorities say 108 people were hurt.

Seventeen-year-old Sameer had gone to the shrine along with his friends. One friend died at the scene and Sameer was in the trauma unit unable to move his broken legs.

"I remember a loud noise and people everywhere were dying. I saw legs and arms and I dragged myself away, hoping for help," he said.

Maj. Gen. Sher Afghan, of the Frontier Corps in Baluchistan, said the bomber had strapped about six kilograms of explosives to his body. The explosive device was packed with ball-bearings and nine-millimeter bullets, he said.

At the mortuary, grieving family members discussed the IS claim of responsibility.

Mohammed Aslam questioned the claim as he searched for his 15-year-old daughter Fatima.

"How can a Muslim ever say that he did something like this. He is not a Muslim," said Aslam.

Ambulances carrying more bodies continued to arrive. One worker, Mir Hasan, could barely keep his red eyes open as he carried another body on a stretcher into the ice-cold room to wait to be claimed.

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Associated Press writer Adil Jawad contributed to this report.