The death toll from a suicide bombing at a mosque in northwestern Pakistan rose to 88 on Tuesday, officials said. The assault, on a Sunni mosque inside a major police facility, was one of the deadliest attacks on Pakistani security forces in recent years.

More than 300 worshippers were praying in the mosque in the city of Peshawar, with more approaching, when the bomber set off his explosives vest on Monday morning. The blast ripped through the mosque, killing and injuring scores and also blew off a part of the roof.

What was left of the roof then caved in, injuring many more, according to Zafar Khan, a police officer. Rescuers had to remove mounds of debris to reach worshippers still trapped under the rubble.

More bodies were retrieved overnight and early Tuesday, according to Mohammad Asim, a government hospital spokesman in Peshawar, and several of those critically injured died. “Most of them were policemen,” Asim said of the victims.

Bilal Faizi, the chief rescue official, said rescue teams were still working Tuesday at the site as more people are believed trapped inside. Mourners were burying the victims at different graveyards in the city and elsewhere. The bombing also wounded more than 150 people.

It was not clear how the bomber was able to slip into the walled compound in a high-security zone with other government buildings and get to the mosque — an indication of a major security lapse.

The investigation will show “how the terrorist entered the mosque,” said Ghulam Ali, the provincial governor in the Khyber Pakhtunkhwa province, where Peshawar is the capital.

“Yes, it was a security lapse," he added.

Prime Minister Shahbaz Sharif visited a hospital in Peshawar after the bombing and vowed “stern action” against those behind the attack.

Read more: Suicide bomber kills 59, wounds over 150 at Pakistan mosque

“The sheer scale of the human tragedy is unimaginable. This is no less than an attack on Pakistan,” he tweeted. He expressed his condolences to the families of the victims, saying their pain ”cannot be described in words.”

Authorities have not determined who was behind the bombing. Shortly after the explosion, Sarbakaf Mohmand, a commander for the Pakistani Taliban — also known as Tehreek-e-Taliban Pakistan or TTP — claimed responsibility for the attack in a post on Twitter.

But hours later, TTP spokesperson Mohammad Khurasani distanced the group from the bombing, saying it was not its policy to target mosques, seminaries and religious places, adding that those taking part in such acts could face punitive action under TTP’s policy. His statement did not address why a TTP commander had claimed responsibility for the bombing.

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.

Earlier this month, the Pakistani Taliban claimed one of its members shot and killed two intelligence officers, including the director of the counterterrorism wing of the country’s military-based spy agency Inter-Services Intelligence. Security officials said Monday the gunman was traced and killed in a shootout in the northwest, near the Afghan border.

The TTP is separate from but a close ally of the Afghan Taliban. It has waged an insurgency in Pakistan in the past 15 years, seeking stricter enforcement of Islamic laws, the release of its members in government custody and a reduction in Pakistani military presence in areas of Khyber Pakhtunkhwa province it has long used as its base.

The Pakistani Taliban are the dominant militant group in the province, and Peshawar has been the scene of frequent attacks. In 2014, a Pakistani Taliban faction attacked an army-run school in Peshawar and killed 154, mostly schoolchildren.

The regional affiliate of the Islamic State group has also been behind deadly attacks in Pakistan in recent years. Overall, violence increased since the Afghan Taliban seized power in neighboring Afghanistan in August 2021, as U.S. and NATO troops pulled out of the country after 20 years of war.

The Pakistani government’s truce with the TTP ended as the country was still contending with unprecedented flooding last summer that killed 1,739 people, destroyed more than 2 million homes, and at one point submerged as much as a third of the country.

The Taliban-run Afghan Foreign Ministry said it was “saddened to learn that numerous people lost their lives" in Peshawar and condemned attacks on worshippers as contrary to the teachings of Islam.

U.S. Secretary of State Antony Blinken, who is on a visit to the Middle East, tweeted his condolences, saying the bombing in Peshawar was a “horrific attack."

“Terrorism for any reason at any place is indefensible," he said.

Condemnations also came from the Saudi Embassy in Islamabad, as well as the U.S. Embassy, which said that the “United States stands with Pakistan in condemning all forms of terrorism.”

U.N. Secretary-General Antonio Guterres called the bombing “particularly abhorrent” for targeting a place of worship, U.N. spokesman Stephane Dujarric said.

Cash-strapped Pakistan faces a severe economic crisis and is seeking a crucial installment of $1.1 billion from the International Monetary Fund — part of its $6 billion bailout package — to avoid default. Talks with the IMF on reviving the bailout have stalled in the past months.

Former Pakistani Prime Minister Imran Khan also expressed his condolences, calling the bombing a “terrorist suicide attack.”

Sharif’s government came to power in April after Khan was ousted in a no-confidence vote in Parliament. Khan has since campaigned for early elections, claiming his ouster was illegal and part of a plot backed by the U.S. Washington and Sharif dismiss Khan’s claims.