With peace talks largely abandoned and leaders of both sides trading accusations, Thursday's violence, which injured at least 78 people, pushed this tropical island nation further toward outright war.
The dead included at least 15 schoolchildren, their blue uniforms coated with blood and gore as authorities lined up corpses on the floor of a nearby hospital so relatives could identify them.
The rebels, a secretive but highly organized and well-armed movement that has been fighting for two decades to create a homeland for the country's minority Tamils, denied any responsibility for the attack, suggesting it was done by shadowy forces they say are trying to create unrest.
But officials insisted Liberation Tigers of Tamil Eelam rebels were responsible.
"This is a barbaric act of the LTTE," said government spokesman Keheliya Rambukwella. "Their aim is to provoke a backlash."
The blast was caused by a pair of mines hanging from a tree and detonated from a remote position, said military spokesman Brig. Prasad Samarasinghe. Rigging explosives to trees or bicycles is a common Tiger tactic, officials say, done so the ground does not absorb as much of the force of a blast.
Police said most of the victims were ethnic Sinhalese, the island's majority ethnic group, and that many were traveling to the funeral of a policeman killed Monday in another attack believed to have been committed by the rebels.
Officials said it was not clear if the bus was targeted because many passengers were going to the funeral.
Late Thursday afternoon, the bus remained knocked on its side as heavily armed soldiers stood nearby, its roof stained with blood while shoes, a mobile phone and women's purses were scattered around.
At the hospital, relatives could barely contain their grief.
"They are all gone," screamed Bandula Gamini, who lost his wife, son and mother-in-law in the blast. Gamini, who had been unable to get a seat, was standing three rows behind the rest of his family when "there was this blast and I recall being trapped."
Gamini was eventually pulled from the bus by a pair of policemen.
A doctor at the hospital where the victims were taken, S.B. Bothota, said 15 schoolchildren were among the 64 people killed. Another 78 people were wounded, he said.
Meanwhile, dozens of people were fleeing the small villages around where the attack occurred, fearing it could trigger more violence. They were taking shelter in school where soldiers had been deployed to protect them.
Rebel commanders met quickly to map out their next moves.
"Our Central Command is discussing about possible defensive measures we need to take to protect our Tamil people," senior rebel leader Seevaratnam Puleedevan told The Associated Press from the rebel-controlled town of Kilinochchi. He said the air strikes have left "a lot of casualties," but gave no figures.
Samarasinghe confirmed that jets had dropped bombs on rebel positions, but only said the air force was taking "limited" deterrent action.
The Tigers dismissed accusations they were behind the attack, with Puleedevan suggesting the attack could be "the work of forces seeking to create ethnic tension."
"The Liberation Tigers condemn the attack on civilians in strongest possible terms," Puleedevan was quoted as saying by TamilNet, a pro-Tiger Web site.
The Tigers fought for 20 years to carve out a separate homeland for the country's 3.2 million minority Tamils, who are largely Hindu, saying years of oppression by the majority Sinhalese Buddhists left them no other choice.
The fighting left the Tigers with control of large parts of the north and east, where they run their own de facto state.
A cease-fire four years ago ended full-scale fighting, but sporadic violence persisted. There was a brief calm after the 2004 tsunami, which left 35,000 Sri Lankans dead and which many hoped would bring the two sides together, but they quickly fell to squabbling over control of aid money.
The situation grew worse in August, with the assassination of Foreign Minister Lakshman Kadirgamar, a killing blamed on the Tigers.
Four months later, the Tigers killed 12 navy sailors — the first major attack in four years — and the situation on the ground has only deteriorated since then, with violence that has left more than 600 soldiers, rebels and civilians dead.
Both sides have blamed the other for the violence, and the Tigers also routinely blame a breakaway rebel faction for attacks on civilians.
Diplomatic efforts to quell the violence and get the peace process back on track have not fared much better.
The Tigers pulled out of peace talks in April, and then last week scuttled negotiations by refusing to meet representatives of the government side after arriving in Oslo, the venue for the talks.