MANILA, Philippines – The death toll from a powerful roadside bombing in Cotabato, Mindanao doubled overnight to eight Tuesday (August 6th), with President Benigno Aquino vowing to find the "terrorists" responsible, AFP reported.

Four people were killed at the scene Monday and four more died in hospital, officials said. Aquino suggested the blast may be linked to groups opposed to the government's continuing peace talks with Muslim rebels.

Earlier, city police chief Rolen Balquin said investigators were looking into whether the target was a convoy carrying a sister of Cotabato mayor Japal Guiani. "Our initial investigation is leading to that conclusion," he said.

Cynthia Sayadi, the mayor's sister who is also the city administrator, suffered minor injuries. Two of Sayadi's bodyguards were among those killed.

Balquin said none of the main armed groups on Mindanao have claimed responsibility for the attack.

Residents reported Cotabato, a mixed-Muslim-Christian city of 300,000, was tense Tuesday amid increased patrols by police and marines and additional checkpoints.