A grenade explosion killed two people and wounded at least 30 others outside a crowded bar in the southern Philippines, officials said Sunday.

Investigators were trying to identify the attackers who tossed the grenade late Saturday outside the El Sentro Resto Bar and a nearby drug store in Iligan city. Police found the pin of the grenade, which also damaged a number of parked cars and sent people fleeing in panic, army Col. Daniel Lucero said.

It remained unclear if Muslim rebels, al-Qaida-linked militants or extortion gangs were behind the blast or if it was sparked by feuding civilians, Lucero said.

President Benigno Aquino III called on the attackers to surrender, warning that authorities would not stop until they were captured. He said investigators were looking at two motives for the attack but did not elaborate.

The government of Australia, which funds anti-poverty projects in the south, expressed its sympathies to the victims.

Iligan, an industrial and trading center 780 kilometers (485 miles) southeast of Manila, has been targeted by Muslim separatist rebels in the past.