No one was hurt in the attack, which happened while YBL bus number 7508 was maneuvering towards its main terminal at around 5:45 p.m.
The bomb, which reportedly contained incendiary materials packed inside a box, exploded at the rear portion of the bus, ripping off its window and burning several seats.
Senior Supt. Robert Kiunisala, South Cotabato police director, said the bomb attack was claimed by the Al-Khobar group, an alleged band of extortionists that had also claimed the recent attacks on passenger buses in Cotabato City and Bansalan town in Davao del Sur province that killed 10 persons.
He said an alleged member of the gang initially called up the YBL office through mobile phone declaring that a bomb was planted by the group in one of its buses and that it would explode anytime.
"The bomb went off several moments after that phone call," Kiunisala said.
Kiunisala said the explosive was left underneath the second to the last seat by two men who disembarked from the bus in front of a shopping mall three blocks away from the explosion site.
"It’s just fortunate that the passengers have already disembarked when the explosion happened," he said, adding that the contents of the explosive were made to kill and maim people.
Olimpio Par, YBL operations manager, said the suspects were reportedly among the passengers who took the bus when it stopped over the Tacurong City terminal while en route to this city.
The bus plies the routes from Koronadal City to Tacurong City and Isulan town in Sultan Kudarat.
"Our conductor actually noticed that they were already acting differently. They seemed uneasy and pale he unfortunately did not mind them," he said.
He said one of the passengers reportedly noticed that one of the perpetrators was grinning while he was going down from the bus in front of the KCC Mall here.
Par said the perpetrator, who was wearing black tee-shirts and white trousers, is slim in built, clean-shaven, dark complexion and believed to be in his early 20s. The witnesses failed to notice the appearance of the other bomber.
Par said it was possible that one of the culprits was the one who called up their office to inform about the bomb at least three minutes before it exploded.
He said the caller declared to a female YBL employee who took the call that a bomb was planted at bus number 7508 and that authorities could no longer stop it from exploding anytime.
Par confirmed that the company, one of the biggest bus operators in Southwestern Mindanao area, received a call two weeks ago from a man claiming to be a member of the Al-Khobar group.
He said the caller demanded a hefty amount and reportedly warned that the group would reportedly attack their buses if they refuse to cooperate.
Meantime, South Cotabato Gov. Daisy Avance-Fuentes admitted that she is worried over the effect of the attack on the province's week-long 8th T'nalak Festival and 41st Foundation Anniversary celebration that will open on July 12.
"I would like to assure everyone that we are in control of the situation and our celebration will go on as planned. This is an isolated case and I would like to assure our visitors that we have properly secured the entire province," she said.
Fuentes hinted that authorities are already pursuing several leads regarding the identity of the suspects and their location.
"We're hoping for a major breakthrough soon and we're praying that all these efforts would be positive," she added.