Muralidhar Reddy, the Sri Lanka correspondent of India's Hindu newspaper, told Al Jazeera from Colombo: "The latest is that this bus was coming from the military headquarters, which is about a kilometre away from the blast site."
The military base holds both the air force and army headquarters.
The hospital director said most of the victims were civilians.
A similar roadside bomb killed 16 people on December 5 in the north-central district of Anuradhapura.
Another 19 people were killed in a bomb attack outside a clothing store in the Colombo suburb of Nugegoda on November 28.
"We have nothing to do with that," Rasiah Ilanthiraiyan, a spokesman for the separatists Liberation Tigers of Tamil Eelam (LTTE), based in Kilinochchi, said on Wednesday, referrring to Wednesday's bombing.
"It is up to the government to find out who did it."
He said the Tigers had noticed a military build-up on the government side of heavily defended forward defence lines that separate state from rebel-held territory in the north.
"What can I say? We can see more clashes in the future," he said.
Wednesday's attack came the day after an assailant shot dead a prominent Tamil MP inside a Hindu temple in Colombo.
The main opposition group, United National Party (UNP), on Tuesday charged the government with T Maheshweran's death, saying it failed to provide adequate security.
Maheshweran survived a similar shooting three years ago while running for general elections.
He is a former Hindu affairs minister and the UNP's chief district organiser in the northern Jaffna peninsula.