At least six people have been killed and 20 others injured after a car bomb exploded close to the mayor's office in the Colombian port city of Buenaventura.
The blast, which occurred on Wednesday and was the worst attack in Colombia this year, destroyed part of the chief provincial proescutor's office.
At least six of the injured worked in that office, officials said.
One of those killed in the blast was a police officer who managed to help evacuate the site of the blast before succumbing to his wounds, police said.
Footage on local television showed wrecked taxis and destroyed shop fronts as residents carried wounded people to hospitals minutes after the blast.
General Freddy Padilla, the head of Colombia's armed forces, blamed fighters from the Revolutionary Armed Forces of Colombia, known as Farc, a Marxist armed group that has been battling the government in Bogota for more than four decades.
"Surely this was the Farc," he said.
But Alvaro Uribe, Colombia's president, said it was too early to lay blame as police had not yet announced the type of explosive used.
Uribe announced a reward of up to $150,000 for information leading to those responsible.
"We can't let our guard down," he told reporters in Medellin before heading to Buenaventura.
Guillermo Mendoza, Colombia's chief prosecutor, said he would not rule out retaliation against his office by drug traffickers, or a fight between rival gangs.
No group has claimed responsibility for the blast.
Buenaventura is Colombia's largest port, handling half of Colombia's coffee exports, but one of the country's poorest cities and a major drug trafficking route to the Pacific coast.
Uribe, who leaves office in August, has put the Farc on the defensive since taking office in 2002 and it is estimated their forces have been halved, to about 6,000-10,000 fighters.