MANILA - Philippine authorities have identified two people of interest for their possible involvement in Sunday's blast inside a southern Philippines' university gymnasium that killed at least four and injured some 50 others, military and police officials said Monday.

The bombing targeted students and teachers of the Mindanao State University in Marawi City, Lanao del Sur province on Mindanao island, who attended a Catholic service at 7 am local time Sunday.

Allan Nobleza, regional police chief of the Bangsamoro Autonomous Region in Muslim Mindanao (BARMM), told a TV interview that the military had two people of interest with links to a local extremist group.

Ricardo de Leon, director general of the Philippines' National Intelligence Coordinating Agency, said in a radio interview that authorities are questioning the duo for their suspect involvement in the bombing.

Both officials declined to identify the two people of interest and their affiliated group, saying the investigation is ongoing.

In 2017, local terrorists pledging support for the Islamic State, including a faction of the Abu Sayyaf Group, the Maute Group, among others, occupied and held Marawi, a lakeside city, for five months, resulting in over 1,200 deaths, hundreds of thousands of residents displaced, and many parts of the city destroyed.

However, in a statement issued hours after the attack, Philippine President Ferdinand Romualdez Marcos blamed "foreign terrorists" for the bombing, without giving further details.

Philippine Defense Secretary Gilbert Teodoro also told a news conference that "there are strong indications of a foreign element" in the attack.

According to Chief of Staff of the Armed Forces of the Philippines General Romeo Brawner, the bombing could be a retaliatory attack by extremists in the wake of a series of military offensives in recent days.

The Philippine troops killed 11 alleged members of Dawlah Islamiya, an Islamic militant group, on Friday in a clash in Maguindanao del Sur province on Mindanao island.