The death toll from a massive suicide bombing that targeted an election rally for a pro-Taliban Muslim leader has risen to 54, as Pakistan held funerals and the government promised to hunt down those behind the attack.

While no one immediately claimed responsibility for Sunday’s bombing, which also wounded nearly 200 people, police said their initial investigation suggested that the ISIL (ISIS) regional affiliate could be responsible.

The victims were attending a rally organized by the Jamiat Ulema-e-Islam party, headed by hardline politician and Muslim leader Fazlur Rehman. He did not attend the rally, held under a large tent close to a market in Bajaur, a district in Khyber Pakhtunkhwa province that borders Afghanistan.

Victims of the bombing were buried in Bajaur on Monday.

Rehman, who has long supported Afghanistan’s Taliban government, escaped at least two known bomb attacks in 2011 and 2014 when bombings damaged his car at rallies.

As condolences continued to pour in from across the country, dozens of people who received minor injuries were discharged from hospital while the critically wounded were taken to the provincial capital of Peshawar by army helicopters. The death toll continued to rise as critically wounded people died in hospital, physician Gul Naseeb said.

On Monday, police recorded statements from some of the wounded at a hospital in Khar, Bajaur’s largest town. Feroz Jamal, the provincial information minister, said police were “investigating this attack in all aspects”.

The death toll rose to at least 54 on Monday, as around 90 injured people were still being treated at hospitals in the northeastern province of Khyber Pakhtunkhwa, rescue official Bilal Faizi said.

Provincial police chief Akhtar Hayat Khan confirmed the explosion was caused by a suicide bombing, and that DNA testing was being conducted to identify the bomber.

Local police chief Nazir Khan said that at least three suspects were arrested overnight in a possible link with the bombing and were being interrogated by the intelligence and law enforcement agencies.

“What we have concluded from the initial investigation is that the attack carries the hallmark of Daesh [ISIL],” Khan said.

Sunday’s bombing was one of the four worst attacks in northwestern Pakistan since 2014, when 147 people, mostly schoolchildren, were killed in a Taliban attack on an army-run school in Peshawar.

Al Jazeera’s Kamal Hyder, who was reporting from the bombing site in Bajaur, said that had the explosion happened in a more confined space, the “casualties, although high, would have been much higher”.

Hyder added that the bomber “was able to get into this area with ease because most of the people are in local dress”, and concealing explosives under the traditional shalwar kameez.

The regional ISIL affiliate in Khorasan province is based in neighbouring Afghanistan’s Nangarhar province and is a rival of the Afghan Taliban. Bajaur was a stronghold of the Pakistani Taliban – a close ally of Afghanistan’s Taliban government – before several Pakistani army offensives that ended in 2016 claimed to have driven them out of the area.

The Muslim leader’s supporters had gathered in Bajaur on Sunday as part of their party’s preparations for the next parliamentary elections, expected sometime in October or November after the current parliament’s five-year term ends.

Bajaur has previously been a hotbed of attacks and was once controlled by the Taliban, according to Nizam Salarzai, executive director of The Khorasan Diary, a local media outlet. Since then, he said, the security situation has gotten significantly better.

But, he explained, the “recent surge in militancy is quite alarming, and especially attacks like these where the Taliban are also condemning it and which are not coming from the Taliban but from other actors as well, it means that the Pakistani state might have to fight on multiple fronts to control this”.

Sunday’s bombing was one of the four worst attacks in northwestern Pakistan since 2014, when 147 people, mostly schoolchildren, were killed in a Taliban attack on an army-run school in Peshawar.

In January, 74 people were killed in a bombing at a mosque in Peshawar. And in February, more than 100 people, mostly policemen, died in a bombing at a mosque inside a high-security compound housing Peshawar police headquarters.

Prime Minister Shehbaz Sharif is expected to dissolve the parliament in August to pave the way for the vote. Rehman’s party is part of Sharif’s coalition government, which came to power in April 2022 by ousting former Prime Minister Imran Khan through a no-confidence vote.

Khan later on Sunday also condemned the bombing, as condolences continued to pour in from across the country.