The Armed Forces of the Philippines (AFP) is convinced that an ISIS-inspired Abu Sayyaf Group is responsible for the twin bombings in the Our Lady of Mount Carmel Cathedral in Jolo, Sulu.
The military is also studying the possibility it was a suicide bombing.
Authorities are faced with several problems in terms of identifying the perpetrators behind the deadly bombing in Jolo on Sunday.
One of the problems they have is that there were no surveillance cameras facing the cathedral directly.
There are, however, two other closed-circuit television cameras (CCTV) that gave them something to think about.
The CCTV footage showed the moment of the first blast. The camera seemed to flicked on impact.
The pedestrians, once calm, are now hurrying off.
But just before the second blast, spotted on the footage were three men who were hurriedly passing something onto each other before hurriedly leaving the scene.
The military identified them as Ajang Ajang -- bandits conducting extortion activities affiliated with the Abu Sayyaf Group.
Military officials said there is reason to believe the man behind the bombing is Abu Sayyaf leader Sawadjaan.
They detonated "pipe bombs" inside and outside the church. These were packed with "ANFO" or ammonium nitrate with fuel, and possibly detonated with a cellphone.
"There is an attempt to bring this back to a religious war. This is an act of terrorism. Hindi religious war ito," Defense Secretary Delfin Lorenzana said.
On Monday, the Islamic State claimed credit for the blasts.
If that were so, it would mean the bomber is among the fatalities.
Twenty bodies have officially been identified and claimed, but according to Lorenzana, there could be one more person unaccounted for.
"In addition to the 20, mayroon pang mga parts ng katawan diyan na nakita which is now with the PNP-SOCO," he said.
The AFP said ANFO was also used in the suicide bombing of a Citizen Armed Force Geographical Unit (CAFGU) detachment in Basilan in July 2018.
President Rodrigo Duterte on Monday afternoon flew to Jolo to personally visit the blast site. He quietly walked the bloodstained floor and looked up at the ceiling that blew off on impact.
Facing the families of the dead, he made it clear who was to blame.
"Ang pangyayaring yun, walang may gusto niyan. Ang may gusto yung talagang pumapatay. Sa tingin ko, katagal-tagal ko na sa gubyerno, talagang mainit ang ISIS," Duterte said.
In many years that the Philippines has had to battle terrorism, there was never been a true suicide bomber until 2018.
Having another proven suicide attack could point to a greater presence or influence of the much more radical terror groups from abroad.