Manila, Dec 4 : Philippine authorities have identified two persons of interest for their possible involvement in the deadly blast inside a gym at the Mindanao State University in Marawi City, Lanao del Sur province, that killed at least four people and injured some 50 others, military and police officials said on Monday. According to the police, the blast occurred at around 7 a.m. when students and teachers had gathered for a Catholic mass inside the gym, reports Xinhua news agency. Allan Nobleza, regional police chief of the Bangsamoro Autonomous Region in Muslim Mindanao (BARMM), told the local media 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 a statement issued hours after the attack, Philippine President Ferdinand Marcos Jr. blamed "foreign terrorists" for the bombing, without giving further details. Defence 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 December 1 in a clash in Maguindanao del Sur province on Mindanao island. In 2017, local terrorists pledging support for the Islamic State, including a faction of the Abu Sayyaf Group, occupied and occupied the lakeside Marawi city for five months, resulting in over 1,200 deaths and hundreds of thousands of residents displaced. Nearly 80 per cent of the country's 113 million population are Catholic and it is not uncommon for school gymnasiums and even shopping malls to designate areas for Sunday Mass, especially in places where there are no churches. /IANS