After the blast, the police placed the entire city under red alert, intensifying security in various entry points to the city as well as in government buildings and crowded places such as public markets, malls and schools.
"We are on red alert because of the bomb explosion," said Senior Superintendent Getulio Napeñas, South Cotabato police director.
He could not yet say who was responsible for the bombing.
The wounded victims were identified as tricycle drivers Rey Nobleta and Renato Bayona, and beautician Joylene Magbanua. Nobleta is in critical condition.
Based on reports gathered by The STAR, a man, described to be about 30 to 40 years old, asked Bayona if he could leave his carton box with him while he bought something inside the public market. Two minutes later, the bomb exploded.
Magbanua was standing near the tricycle, where the bomb went off, while Bayona was a few meters away.
Mayor Fernando Miguel, who immediately went to the blast site, condemned the bombing "as the work of beasts who want death and destruction in the city."
Four bombings had rocked the city, three of them in the public market, from March to July 2003, killing 13 people and wounding more than 100 others.
A supposed special operations group of the Abu Sayyaf, led by a certain Abu Sulaiman, initially claimed responsibility for the attacks.
The police filed murder and multiple murder charges against a dozen people for the bombings, but only five have been arrested.
Three of the suspects — Edris Sapal, Junie Mangadta and Tayuan Dimaloloy — are detained at the South Cotabato provincial jail.