MANILA, Philippines  - Two people were injured after unidentified assailants fired mortar shells in Barangay Poblacion, Ampatuan town in Maguindanao province Sunday evening.

Ampatuan Mayor Zacarias Sangki said the shelling happened at around 7 p.m., causing slight damage to two local houses.

In an interview, Sangki said the mortar shelling could be retaliation for the decision of his son, Vice-Mayor Rasul Sangki, to testify in the Maguindanao massacre case.

The younger Sangki earlier tagged members of the powerful Ampatuan clan for perpetrating the massacre of 57 people in Ampatuan, Maguindanao last November 23. The incident is considered the worst incident of election-related violence in the country's history.

The Ampatuan mayor said he will leave it to law enforcers to investigate the incident. Local police said the incident could be the handiwork of private militiamen loyal to the Ampatuans.

Before the shelling, bomb disposal experts were able to dismantle an improvised explosive intended for a Phoenix gasoline station owned by Sangki. The bomb was given to a gasoline boy, saying that it was a package sent by an unknown person.

The station's personnel did not open the box but instead, put it under a coconut tree across the street. Bomb disposal experts later found the box contained a rocket propelled grenade, batteries, a blasting cap and a cellphone, which would be used as a triggering device.