Pakistani law enforcement agencies say they have made significant progress in the investigation into a deadly suicide bombing in the capital, Islamabad, on Saturday, as authorities continue nationwide operations to dismantle the network behind the attack.

According to official sources, the brother-in-law of the suicide bomber, identified as Yasir, son of Bahadur Khan, was arrested in the southern port city of Karachi. The bomber’s brother has also been taken into custody in Peshawar, in the country’s northwest.

Meanwhile, a key facilitator allegedly linked to the attack was killed during a security operation in the Nowshera district, officials said.

In a major development, security agencies have also arrested the mother of the suicide bomber from a residence in an upscale sector of Islamabad. Authorities described the arrest as one of the most significant breakthroughs in the investigation so far.

The attack occurred on Friday when a suicide bomber struck a Shia mosque on the outskirts of Islamabad during weekly prayers. At least 31 people were killed and 169 others wounded, according to city officials, making it the deadliest attack in the capital since the 2008 bombing of the Marriott Hotel.

The explosion took place at the Imam Bargah Qasr-e-Khadijatul Kubra mosque in the Tarlai area, where large numbers of worshippers had gathered for Friday prayers. Security officials said the attacker was stopped at the mosque gate before detonating explosives.

The Islamic State group (IS) claimed responsibility for the attack, saying one of its militants targeted the congregation by detonating an explosive vest. The claim was reported by the SITE Intelligence Group, which monitors militant communications.

Prime Minister Shehbaz Sharif condemned the attack and vowed that those responsible would be brought to justice. Deputy Prime Minister Ishaq Dar described the bombing as a “heinous crime against humanity” and said Pakistan stood united against terrorism.

The United Nations Secretary-General, António Guterres, also condemned the attack, saying assaults on civilians and places of worship were unacceptable.

The bombing comes amid rising militant violence in Pakistan, particularly in provinces bordering Afghanistan. Pakistan, a Sunni-majority country, has a significant Shiite minority, estimated at between 10 and 15 percent of the population, which has been targeted in sectarian attacks in the past.

Islamabad has accused militant groups operating in Afghanistan of using cross-border safe havens to launch attacks, allegations repeatedly denied by Afghanistan’s Taliban-led government. Tensions between the two countries have increased in recent months, with frequent border clashes reported.

The last major attack in Islamabad occurred in November, when a suicide bombing outside a court killed 12 people. Elsewhere in Pakistan, recent attacks in the southwestern province of Balochistan have killed dozens of civilians and security personnel, prompting large-scale counterterrorism operations.