Thirty-six people were caught in yesterday's roadside blast, with four killed at the scene and another four dying in local hospitals overnight, city police chief Rolen Balquin said. Earlier, the total number of people caught in the blast had been given as 32.
Police investigators are looking at the possibility that the deadly attack was aimed at a convoy carrying a sister of Cotabato city mayor Japal Guiani, the police chief added.
He said none of the main armed groups in the southern island of Mindanao, a hotbed to decades-old rebellions by minority Muslims, have claimed responsibility for the attack.
Police interviewed Cynthia Sayadi, the mayor's sister who is also the city administrator, at her hospital bed where she was treated for minor injuries, said Balquin, who did not disclose what she told blast investigators.
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"Isn't it obvious? Do I have to state the obvious?" Sayadi said in an interview aired by Manila television network ABS-CBN, apparently supporting the police theory.
Two of Sayadi's bodyguards were among those killed in the bomb, which was apparently planted in a vehicle parked at the side of the road, Balquin said.
However, he said the authorities were not ruling out the possible involvement of Islamic militants or groups opposed to ongoing peace talks between Manila and the Moro Islamic Liberation Front (MILF), Mindanao's main Muslim rebel group.
The attack came a month after the United States, Australia and Canada warned their citizens against travelling to Cotabato and two other Mindanao cities.
"We have intensified visibility patrols and are closely monitoring the activities of various threat groups," Balquin said.
Cotabato, a Christian-Muslim entrepot of 300,000 people, was tense today as police and Philippine Marines stepped up patrols and roadside checkpoints, residents said.