The authorities said they had found a body at the scene that they believed was that of a suicide bomber. They also said they had found remnants of an explosives belt.
The Islamic State group on Wednesday claimed responsibility for the attack in a statement posted online.
During the day, a small crowd gathered at the end of the main boulevard here in the capital to pay respects to the dead, laying flowers, draping the national flag and standing vigil as investigators in white overalls continued to examine the crime scene.
“Everyone is shocked,” said Habiba Lamouchi, 50, a homemaker who came to pay her respects. “I am not scared — we are courageous people — but people are sad,” she said.
The blast, which also wounded 20 others, prompted the government to declare a state of emergency, temporarily close the airport and tourist sites and impose a curfew. Officials also closed Tunisia’s southern border with Libya for 15 days beginning on Wednesday.
The National Security Council of Tunisia also belatedly approved the release of money in a counterterrorism fund that would finance relief projects in the most disadvantaged regions of the country, which is wedged between Libya to the east and Algeria on the west.
Tunisia is the lone country that, after the Arab Spring uprisings, established a genuine democratic transition by adopting a new Constitution in 2013 and holding its first national presidential elections in 2014. But the transition has been marred by attacks by extremist Islamist groups, including two this year that killed more than 60 people at the National Bardo Museum in Tunis and at a resort hotel in Sousse, about 87 miles south of Tunis. Many of the victims were foreign tourists.
The bodies of victims on Tuesday, who were boarding a bus that was to take them to work, have been identified. Their coffins were laid in state under red Tunisian flags at the presidential palace on Wednesday.
A 13th body found at the scene has not yet been identified and is suspected to be that of the bomber, the news media in Tunis reported.
An unnamed official with the union representing the presidential guard told the popular Radio Mosaïque FM that witnesses had seen a young man trying to board the bus ahead of others and that when he was confronted, he detonated his bomb.
The explosion destroyed the bus and blew out windows in a high-rise building nearby.
Ms. Lamouchi, the homemaker, said the bomber was “a coward, not a Muslim, not educated.”
“We don’t know him,” she added. “Our religion does not allow this.”
The blast was the third major attack by militants in Tunisia this year and the first serious assault on the security forces in the capital since jihadists began a campaign of violence in 2011.