The toll from the blasts at a gathering of the Christian sect Jehovah’s Witnesses in Kerala’s Kochi rose to three as a 12-year-old girl succumbed to burn injuries during treatment on Monday, officials said. The girl was a resident of the Ernakulam district, who suffered 95% burns and was on ventilator support.

“As per the medical board, she was given all possible treatment. But subsequently, she stopped responding to the medicines and passed away at 12:40am on Monday,” said a statement from Government Medical College Hospital, where the girl was under treatment.

Two women, Leona, 55, and Kumari, 53, were killed and 52 others injured in improvised explosive device (IED) blasts at the Zamra International Convention and Exhibition Centre at Kalamassery in Kochi around 9:30am on Sunday on the final day of a three-day zonal conference of the Jehovah’s Witnesses, which has a few hundred thousand followers in Kerala.

A man named Dominic Martin surrendered to the police hours after the blasts, claiming responsibility for the explosions. In a live social media video, he said he was a member of the Christian sect and “reacted” against it as he believed its teachings “were anti-national and seditious”. Martin said he started getting “angered” by the Jehovah’s Witnesses teachings six years ago and that there was “something wrong” with the group. He claimed to have asked them to stop “promoting” hatred and sedition. “I asked them several times to correct their ways. But they were not ready to do it,” he said

The blasts put authorities across the country on alert as teams from the National Investigation Agency (NIA) and National Security Guard rushed to the scene within hours. They ripped through the convention centre around 9.30am when nearly 2,500 people were in prayer. Survivors said smoke filled the centre after explosions near the spot, where the audio console was kept for Jehovah’s Witnesses annual event marked by prayers, talks and symposiums. The theme for this year’s event was patience.

Kerala health minister Veena George said 17 of the injured were admitted to the intensive care units and the 12-year-old among them was critical.

A preliminary probe into the blasts suggested a timer-based low-intensity IED was used to target the congregation. An NIA team was likely to take over the probe soon. Investigators said that they found batteries and wires that establish the use of a timer. A forensic examination was expected to detail the type of explosive used. The IED was placed near an oil container to exacerbate the fire.

Police were questioning Martin and examining evidence of his involvement in the blast. A preliminary First Information Report did not name a suspect.

Chief minister Pinarayi Vijayan, who promised a thorough investigation, has called for an all-party meeting on Monday over the attack.