He said the suicide bomber, who may have been only 14 and who was beheaded by the blast, had a broken piece of a cyanide capsule embedded in his neck. Tamil rebels wear cyanide capsules around their necks to swallow in the event of capture.
The separatist rebels, who have fought for 10 years for their own homeland, have denied involvement.
Twenty-four people, including the president, died in the explosion on Saturday. Thirty-eight others were injured. The dead included seven police officers, civilian and party supporters.
Yesterday hundreds of people queued for hours to pay their last respects to Premadasa, known as 'the common man's president'. Many elderly women collapsed in tears when they reached his flag-draped coffin. But elsewhere firecrackers were set off to celebrate his death.