The suicide bomber who killed more than 100 people at a mosque in a police compound in the Pakistan city of Peshawar this week wore a police uniform and entered the high security area on a motorbike, a provincial police chief said on Thursday.

The bomber behind Monday’s attack had been identified as a member of a militant network, Moazzam Jah Ansari, police chief of Khyber Pashtunkhwa province, told reporters without giving further details.”I admit this was a security lapse. My men could not stop it. This is my fault,” Ansari said.

The bombing was the deadliest in a decade to hit Peshawar, a northwestern city that has suffered decades of Islamist militant violence and is located near the restive Pashtun tribal lands bordering Afghanistan.

It took place as hundreds of worshippers gathered for noon prayers at a mosque that was purpose built for the police and their families inside the high-security Police Lines district. Ansari said the CCTV footage showed the bomber, wearing a helmet and a mask, riding his motorbike through the main checkpoint of Police Lines.

He then parked his bike, asked directions to the mosque and walked there, Ansari added.

“The police guards at the main entrance thought he was a member of the force; they didn’t check him,” Ansari said.

A day earlier, the police chief said investigators were not ruling out that the attacker could have had “internal assistance”. Several suspects were in police custody, he said. All but three of those killed were police officers, making it the worst attack on Pakistani security forces in recent history.

Police Lines is a colonial-era, self-contained encampment in that houses middle- and lower-ranking police personnel and their families in the provincial capital. Hundreds of police staged demonstrations across the province to protest the attack.

The most active militant group in the area, the Pakistani Taliban, also called Tehreek-e-Taliban Pakistan (TTP), has recently increased attacks on police in the northwestern province as part of its campaign against the government in Islamabad.

The TTP has denied responsibility for the mosque attack.

Pakistani officials say they suspect a breakaway faction of the TTP called Jamat-ul-Ahrar was involved.

Jamat-ul-Ahrar has claimed responsibility for several major attacks in the region over the years, including the twin suicide bombings at All Saints Church that killed scores of worshippers in September 2013, in what remains the most deadly assault on the country’s Christian minority.