According to the Associated Press, an estimated 300 people were inside the mosque, located in a secure police compound in Peshawar, when a militant blew himself up in one of the deadliest attacks against security forces in the country.

As of Jan. 31, the death toll rose to 88 as rescue workers continued to recover bodies underneath the rubble. More than 150 people were wounded in the attack.

Authorities are investigating how the attacker was able to enter the heavily guarded compound that houses police headquarters and the region’s intelligence and anti-terrorism offices.

After visiting the site of the attack, Pakistani Prime Minister Shehbaz Sharif expressed his condolences to the victims and their families and said the "sheer scale of the human tragedy is unimaginable."

"This is no less than an attack on Pakistan," Sharif said in a tweet published Jan. 30. "The nation is overwhelmed by a deep sense of grief. I have no doubt terrorism is our foremost national security challenge."

"My message to the perpetrators of today’s despicable incident is that you can’t underestimate the resolve of our people," he added.

Sarbakaf Mohmand, a commander of the Pakistani Taliban, claimed responsibility for the bombing in a post via Twitter.

However, shortly after, Mohammad Khurasani, spokesman for the Pakistan Taliban, denied the group’s involvement, and said it is not the organization’s policy to attack mosques or places of worship. However, he did not address Mohmand’s claim, AP reported.

The attack drew condemnation from the Taliban in Afghanistan. Suhail Shaheen, head of the Taliban political office in Doha, condemned the attack on the mosque.

"Places of worship have their sanctity in our religion and none has (the) right to violate it," he wrote on his Twitter account.