The blast took place around 4:40 p.m. local time Monday on Sinsuat Avenue in front of a funeral home, said Senior Superintendent Rolen Balquin, acting city director.
He said police are looking into the possibility the attack was aimed at City Administrator Cynthia Guiani-Sayadi or her brother, Mayor Japal Guiani Jr., The Philippine Daily Inquirer reported Tuesday. Guiani-Sayadi was riding in a vehicle near where the explosion occurred.
"She has been receiving threats. Maybe the suspects were able to monitor her movements," Balquin said of Guiani-Sayadi.
"We received information purporting there are threats on our lives one week before the incident," Guiani said, adding he is offering a reward for information leading to the arrest of the culprits.
Two of Guiani-Sayadi's security escorts -- Samsudin Hari and Yusoph Gadju, her cousins -- died on the spot in the attack. She only received a bruise on her knee inside her bulletproof sports utility vehicle, the Daily Inquirer reported.
Passerby Mama Manambuay was dead on arrival at the Cotabato Regional Medical Center. Two minors, a 9-year-old and a 14-year-old, and two adults -- Harris Unto and Salipudin Sindatuk -- died at the hospital. The names of the minors and the eighth victim were not reported.
Thirty people were injured by the blast and 13 remained in the hospital Tuesday, The Philippine Star reported.