Police said said they suspected the men were trying to rig a device that may have exploded accidentally.
Colombo alert
"All vehicles entering the city [Colombo] were checked," a police spokesman said on Tuesday.
"This was a precautionary measure after yesterday's bomb attack."
More than 5,000 people have been killed in a resurgence of the conflict since December 2005 despite a ceasefire being agreed in February 2002.
Separatist violence in Sri Lanka has claimed more than 60,000 lives so far.
Rising toll
A police commando wounded in Monday's bomb attack died of his injuries on Tuesday, raising to eight the number of people killed in that attack at Ratmalana, near a major airbase.
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."
Police casualties
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"
Sri Lankan
police officer |
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.