Anura Yapa, the media minister, said the blast was the work of the Liberation Tigers of Tamil Eelam (LTTE).
Brigadier Prasad Samarasinghe, military spokesman, also blamed the attack on Tamil Tiger separatists.
Rasiah Ilanthirayan, an LTTE spokesman, denied the Tamil Tigers were responsible for the bomb.
He said: "I only have to say that we have nothing to do with it."
A total of 38 people, including six police commandos, were still in hospital after surviving Monday's bomb blast.
|"It looks like a claymore mine was placed on the roof of a roadside shop and detonated with a remote controlled device as the truck passed" |
Nine commandos were in the truck that was heading towards Colombo from a training facility at Katukurunda.
Hundreds of troops pass through the suburban area just 15km south of the centre of Colombo every day.
"It looks like a claymore mine was placed on the roof of a roadside shop and detonated with a remote controlled device as the truck passed," a police officer at the scene said.
The owner of the shop was telling police that the premises had been broken into when the blast ripped through, he said.
The bombing came just days after an army bus was targeted near the entrance to Colombo port, killing one soldier and injuring six other people, and a fierce naval battle off the Jaffna peninsula on Thursday.