Sri Lankan Police detained two people over an attempt to assassinate the president's brother, officials said, as 11 people were killed in fresh fighting.
The Police Criminal Investigations Department took into custody two men who owned the auto-rickshaw used in Friday's suicide bombing targeting Defence Secretary Gotabaya Rajapaksa's heavily armed convoy in Colombo.
"We have tracked down the owner of the vehicle used in the bombing," a police official said. "Two are being questioned."
Forensic experts suggested the three-wheel rickshaw was fitted with eight to 10 kilograms of plastic explosives and packed with ball bearings, which acted as pellets, he said.
Two army bodyguards of Rajapaksa died in the attack, which also wounded 15 people, including five civilians.
The suicide bomber's body was found on the back of a pickup truck caught up in the powerful blast on the high-security Dharmapala Mawatha road. The head was destroyed beyond recognition, police said.
Within hours of Friday's blast, two policemen were killed in a Claymore mine attack in northern Jaffna peninsula, police said, adding that troops killed eight Tiger rebels in two separate clashes in the restive east.
A soldier was also killed in a Tiger grenade attack on troops in Jaffna on Friday, the defence ministry said.
The violence came as Norway's top envoy Jon Hanssen-Bauer visited Colombo for talks with Sri Lankan leaders on reviving a stalled peace bid.
Hanssen-Bauer is due to travel to the rebel-held north of the island Monday.
Despite Friday's attack on defence secretary Rajapaksa, the government said it was still committed to a peaceful solution to the three-decade-old Tamil separatist conflict.
Rajapaksa narrowly escaped death as his bullet-proof limousine was also hit. He drove straight to President Rajapaksa's official residence with only a minor graze above his right eye.
Rajapaksa, the younger brother of President Mahinda Rajapaksa, later showed photographers bloodstains on both sleeves of his white shirt that had come from his wounded guards.
The United States condemned the assassination bid and urged the rebel Liberation Tigers of Tamil Eelam (LTTE) to enter talks.
"The attack bears all the hallmarks of an operation by the LTTE," the US embassy in Colombo said in a statement.
"We once again call on the LTTE to renounce terrorism, to give up violence and to join in negotiating a peaceful solution to Sri Lanka's conflict," it added.
The Nordic truce monitors made a similar appeal to both sides and said the attack had the hallmarks of the Tigers.
The LTTE is fighting for a separate state for Sri Lanka's 2.5 million Tamil minority.
More than 3,400 people have been killed in the past year alone as the truce unravelled and two rounds of peace talks failed.
Over 60,000 people have been killed in the island's separatist campaign since 1972.