Jema'ah Islamiyah (JI), a Southeast Asian extremist group was behind Thursday's mall bombing in the southern Philippines that wounded four people, local police said on Friday.

Central Mindanao regional police chief German Doria said a janitor saw a woman assembling what appeared to be a homemade explosive inside the mall's comfort room, a report by dzBB radio said referring to the bomb explosion inside a shopping mall in Tacurong, Sultan Kudarat Province.

The mall received extortion letters demanding cash two weeks before Christmas, Doria was quoted by the radio as saying.

The officer said that the signatory of the letters claimed to be Abu Sulaiman of the Indonesia-based JI, and threatened to "blow up" the store if its owner refused to pay up.

However, the mall's owner did not inform police about the extortion letter until Thursday's incident.

Police said that the homemade explosive may have been made up of gunpowder inside a soft drink can. Doria has ordered the local provincial and city police to "carefully watch" people buying soft drinks in cans, the report said.

The JI have been linked to the past bombings occurred in Mindanao, the southern Philippine island.

Source: Xinhua