MANILA - Police believe Filipinos were behind an explosion last week which killed three soldiers and three civilians.
If confirmed, it would be the first case of local militants carrying out a suicide attack.
"What we know, and the (military) knows this, (is that) they are Filipinos. They are not foreign nationals," General Oscar Albayalde, head of the 170,000-strong police force, told reporters on Monday (July 1).
However, Defence Secretary Delfin Lorenzana said in a separate news briefing that nothing was conclusive yet.
"Somebody said the bombers were Indonesians, somebody said Filipinos. But let us wait," he said.
If confirmed, the information suggests that the Islamic State's ultra-radical teachings have taken deeper root among Filipino militants.
"Yes, yes, I believe… this has raised the level of extremism here," said Mr Lorenzana.
Last Friday, two militants set off bombs strapped to their bodies at a temporary camp of a special army counter-terrorism unit in Indanan town on Jolo island, in the Philippines' restive south.
Three soldiers who were manning the camp's gate died along with three civilians who were nearby. The two attackers were also killed.
The Islamic State in Iraq and Syria (ISIS) has claimed responsibility for the attack.
A tweet last Saturday by the director of the Site Intelligence Group, which monitors extremist groups, showed a photo of two militants who ISIS claim carried out the attacks.
They appear to be in their 20s, Asian and wearing what appeared to be vests laden with bombs.
There have been two other instances of suicide bombings in the Philippines. But both involved militants from abroad.
Security officials and some analysts have long assumed that local extremist groups like the Abu Sayyaf, although pledging allegiance to ISIS and providing sanctuary to militants from abroad, eschew suicide attacks as a weapon.
In January, an Indonesian couple, with help from a faction of the ISIS-linked Abu Sayyaf group, attacked Jolo's Roman Catholic cathedral, killing 23 people and wounding at least 100.
In July last year, a Moroccan, identified as Abu Kathir Al-Maghribi, detonated bombs hidden inside a van he drove to an army checkpoint on Basilan island, near Jolo. The blast instantly killed a soldier, five paramilitaries, four civilians - including a mother and her child - and the bomber.
Major-General Cirilito Sobejana, head of the Western Mindanao Command, said on Sunday that Al-Maghribi's 14-year-old son could have been one of the bombers in last Friday's attacks on the camp of the 1,500-man First Brigade Combat team, which was especially trained to go after the Abu Sayyaf.
The Abu Sayyaf, founded in the 1990s with seed money from Osama bin Laden's Al-Qaeda network, has been tagged as the top suspect in the attack.
"I think we have a lot of work to do," Mr Lorenzana said on Monday.