MANILA, Philippines – Authorities are looking into extortion as a possible motive in the Atilano Pension House blast in Zamboanga City last Sunday that killed 3 people.

Police Regional Office Director Elpidio de Asis said the explosion could be linked to past incidents in Isabela, Basilan, where 2 improvised explosive devices (IEDs) believed to be planted by the group of Abu Sayyaf leader Puruji Indama were recovered.

De Asis said establishments in Basilan have received extortion letters from Indama's group allegedly asking for revolutionary tax.

The same letter was also sent to an establishment in Sangali, Zamboanga City where an IED exploded killing 1 person.

De Asis said the group is probably still planning to "introduce" themselves to the owner of the pension house Carlos Atilano.

Atilano denied he had communicated with Indama.

Meanwhile, Hussein Ahaddin, an alleged member of Abu Sayyaf, was arrested in Barangay Muti in Zamboanga City on Tuesday, authorities said.

Ahaddin has denied involvement in the explosion. -- Report from Jewel Reyes, ABS-CBN News Zamboanga