MANILA - The brother of a slain Abu Sayyaf leader may have triggered the twin bombs that killed 21 people and hurt at least 80 others at the Jolo Cathedral on Sunday, the military said Tuesday, citing surveillance footage from streets close to the site of the blast.

The CCTV clip showed Alias Kamah, brother of bandit leader Surakah Ingog, wearing a blue-green jacket as he tinkered with a cellphone that could have been used to detonate the bombs in Jolo Cathedral, said Col. Gerry Besana, spokesperson of the military's Western Mindanao Command.

Kamah, a known bomb maker, ran away from the church and out of the video's frame with several cohorts moments after the explosion, Besana said.

The group was also behind several kidnapping and extortion cases in Sulu, he said.

WATCH: CCTV footage showing 3 members of the Ajang-Ajang group fleeing just after the explosions. Authorities believe they are responsible for the twin blasts that happened in Jolo, Sulu. | via @maan_macapagal #JoloBlast pic.twitter.com/VlrHNSYAzJ

— ABS-CBN News (@ABSCBNNews) January 29, 2019

The first bomb went off during an early morning Mass, shattering pews and windows, and leaving bodies strewn inside the Jolo Cathedral. The second bomb exploded as security forces rushed to help the wounded.

The Islamic State on Monday claimed credit for the attack, saying two suicide bombers detonated a belt of explosives, according to US-based SITE Intelligence Group which monitors jihadist activities.

The military was not discounting this claim but said the Ajang-Ajang, a subgroup of the bandit Abu Sayyaf Group, was a prime suspect.

The Jolo bloodshed came a week after a majority of voters approved the creation of the Bangsamoro, a new region giving Muslims in the south more control over their own affairs.

Sulu, which includes the remote island of Jolo, voted against creating the new region, but the province will still be part of the Bangsamoro because voters from the current autonomous region were in favor of it.

On Tuesday, police said the twin blasts was a "clear act of terror" and had nothing to do with the plebiscite.

On Feb. 6, 39 villages in North Cotabato and Lanao del Norte will decide on whether or not to join the Bangsamoro region.

• Military gears for second Bangsamoro vote after Sulu church blasts

-- Report from Maan Macapagal, ABS-CBN News