A suicide bomber detonated explosives during crowded prayers at a mosque inside a police compound in Pakistan on Monday, causing the roof to cave in. At least 34 people were killed and 150 wounded, officials said. Most of the casualties were police officers. It was not clear how the bomber was able to slip into the walled compound, which houses the northwestern city of Peshawar’s police headquarters and is itself located in a high-security zone with other government buildings.

Sarbakaf Mohmand, a commander for the Pakistani Taliban, claimed responsibility for the attack on Twitter. The main spokesman for the militant group was not immediately available for comment. Pakistan, which is mostly Sunni Muslim, has seen a surge in militant attacks since November, when the Pakistani Taliban ended their cease-fire with government forces. Monday’s assault on a Sunni mosque was one of the deadliest attacks on security forces in recent years.

More than 300 worshippers were praying inside the mosque, with more approaching, when the bomber set off his explosives vest. Many were injured when the roof came down, according to Zafar Khan, a local police officer. Rescuers scrambled to remove mounds of debris from the mosque grounds to reach worshippers still trapped under the rubble, police said. Meena Gul, who was inside the mosque when the bomb went off, said he doesn’t know how he survived unhurt. The 38-year-old police officer said he could hear cries and screams after the blast.

Siddique Khan, a police official, said the death toll rose to 34, and the dead included Noor-ul-Amin, the prayer leader. He said the attacker blew himself up while among the worshippers. Peshawar police chief Ijaz Khan said at least 150 were wounded. A nearby hospital listed many of the wounded in critical condition, raising concerns the death toll could still rise.

Peshawar is the capital of Khyber Pakhtunkhwa province, where the Pakistani Taliban have a strong presence, and the city has been the scene of frequent militant attacks. The militant group, also known as Tehreek-e-Taliban Pakistan or TTP, has waged an insurgency in Pakistan over the past 15 years. It seeks the stricter enforcement of Islamic laws, the release of their members who are in government custody and a reduction in the Pakistani military presence in areas of Khyber Pakhtunkhwa province that it has longed used as its base.