Hell broke loose in Pakistan’s north-western city, Peshawar, as a suicide bomber breached security inside a mosque on Monday, killing 100 people and injuring almost 227. The attack, in the Police Lines district, was the deadliest in a decade. According to a medical official, the South Asian country faces a mounting security challenge from armed groups. He added that the majority of those killed were police officials.

The suicide bombing caused the mosque’s roof to collapse, and rescuers had to remove mounds of debris to recover many of the bodies, authorities said. Distraught relatives thronged hospitals in Peshawar on Tuesday to look for their kin a day after a suicide bombing ripped through a crowded mosque in a heavily fortified area of the city. Pakistani authorities scrambled to determine how a suicide bomber could carry out one of the country’s deadliest attacks in years, unleashing an explosion in a crowded mosque inside a highly secured police compound. It is still unclear who carried out the bombing, but counter-terrorism police are investigating how the bomber reached the mosque inside a walled-off police headquarters compound called Police Lines.

The district also includes government buildings. A commander from the Pakistani Taliban, known by the acronym TTP, claimed responsibility, but a spokesman for the group later distanced the TTP from the event, saying it was not its policy to attack mosques.

“Yes, it was a security lapse,” said Ghulam Ali, the provincial governor in the Khyber Pakhtunkhwa province, of which Peshawar is the capital. Akhtar Ali Shah, a former regional interior secretary once based in Peshawar, said it “was not a spur-of-the-moment attack. It was the handiwork of a well-organised group,” he told Al-Ahram. He also said those behind the attack must have had inside help to gain access to the compound and probably entered it several times for reconnaissance or even to plant explosives ahead of time.

“It’s not a security lapse; it’s a security breach,” he said. “From all entry points, there are multiple layers of security you have to cross with ID checks.”

Talat Masood, a retired army general and senior security analyst, said Monday’s suicide bombing showed “negligence.” Speaking to Al-Ahram, CCPO Khan said that there is a possibility that the assailant may have entered the compound in an official vehicle. “ [The] Counter Terrorism Department (CTD) is investigating the case,” he added.

He said that 1,500 to 2,000 personnel come and go daily as there are more than eight units of offices, including FRP, Special Security Unit (SSU), Elite Force and CTD in the Peshawar Police Lines.

“The assailant may have already been present in Police Lines,” Khan said, adding that the nature of the explosion can only be known after the rescue operation is completed.

The hall of the mosque was old, while the rest had been newly built. The Peshawar CCPO further said the mosque’s wall had collapsed, causing more damage. The hall was also damaged by fire after the explosion.

The attack on the mosque occurred at the start of a critical week for Pakistani diplomacy. On Monday, the president of the United Arab Emirates, Mohamed bin Zayed Al Nahyan, was due to visit Islamabad, although the trip was cancelled at the last minute because of bad weather. It also happened a day before an International Monetary Fund (IMF) mission arrived in Islamabad for talks on a stalled $7 billion bailout.

Last March, Peshawar was the target of another bombing, which killed dozens in a Muslim mosque. In the capital Islamabad, police issued a high alert and said security at all entry and exit points to the city had been increased.