The slain councilor, Alexis Costales, 37, who belonged to the Reporma party, and his nephew, Clarence Cang, both died on the spot from multiple gunshot wounds.
Another nephew of Costales, Patrick Cang, 12, was wounded in the attack.
"The death of Kagawad Costales is a big loss to the political community of Cotabato City. It is an eye-opener for us to continue striving hard to promote lasting peace in our communities," said City Mayor Muslimin Sema.
Costales, known for his advocacy on environmental protection, empowerment of women and children and religious solidarity among the city’s culturally diverse people, belonged to a known religious Catholic clan in the city.
Senior Superintendent Sangacala Dampac, city police chief, said Costales and his nephews had just arrived from church on board a red Mitsubishi Lancer car and were about to enter their frontyard when two motorcycle-riding men opened fire with caliber .45 automatics.
"We just have to give our investigators enough time to do their work. We need the cooperation of all his fellow councilors, his relatives and all witnesses to the crime for us to identify the real culprits," Dampac said.
Maj. Gen. Roy Kyamko, commander of the Army’s 6th Infantry Division, said 30 intelligence agents have been fielded in the city’s entry and exit points to help track down the gunmen.
Sema, secretary general of the Moro National Liberation Front, said the city’s 37 barangay chairmen have also been ordered to help gather information on the identities of the killers and their motive for the attack.
Political contemporaries of Costales said the councilor had no known "personal adversaries." His wife and parents refused to talk to reporters.
Sema said he is ready to shell out a "monetary incentive" in exchange for any information that would lead to the arrest of the assassins.