"We are still investigating," said EW Prathapasinghe, a deputy inspector general of police for the island's western province.
"At the moment, 18 people are being questioned, but no one is in custody.
"It was a time bomb weighing about 2kg and fixed onto a seat. This is the work of the LTTE [Liberation Tigers of Tamil Eelam]."
|Friday's bomb exploded 36km north of Colombo|
Keheliya Rambukwella, the government's defence affairs spokesman and the minister of policy planning, said: "The historical evidence points the finger towards the Tigers for the blast.
"These are signs of desperation in view of the defeats they have faced recently in the battlefield".Air raids
The bus attack came as government forces carried out a fourth day of air raids on Tamil Tiger positions in the north of the island.
On Friday, Sri Lankan air force jets destroyed a base belonging to the Sea Tigers, the rebels' naval division, at Wilawattai in the rebel-held Mullaitivu district, Samarasinghe said.
The Tigers have accused the military of non-stop air raids targeting civilians.
Rasiah Ilanthirayan, a spokesman for the Tigers, also accused government forces of killing two state employees and injuring three others in a mine blast on Friday in the Wanni district. The explosion occurred next to a Jeep transporting the officials.
Violence between the sides claimed the lives of more than 3,500 people during 2006.
There was no immediate claim of responsibility or denial from the Tigers, who are fighting for a separate homeland for minority Tamils in the northern and eastern regions of the Sinhalese-majority country.