Four police officers and a civilian were killed when a bomb exploded in a southern city where guerrillas tried to assassinate the mayor with a car bomb two days earlier, authorities said.
The attacks came a week before US President George Bush is due to visit Colombia, Washington's closest ally in the region. Bogota has received more than $4 billion in aid from the US government since 2000 to fight rebels and the drug trade that makes Colombia the world's No. 1 cocaine producer.
Police thought they had deactivated the explosives found on Saturday in a duct at the Neiva city radio station but the bomb blew up as they were moving it.
"We are not sure why it exploded. Everything points to an accident," Defense Minister Juan Manuel Santos said on radio.
Four bomb experts were killed in their vehicle and a female bystander also died, police said.
Rebel attacks are less common since President Alvaro Uribe began an offensive to combat the Revolutionary Armed Forces of Colombia, or FARC, and the cocaine trade the rebels use to sustain Latin America's oldest guerrilla war.
But FARC, Colombia's largest rebel group, remains a potent force in rural areas and often targets local authorities for kidnapping or assassination.
Mayor Cielo Gonzalez, who has been threatened by FARC before, escaped Thursday's attack when one of her bodyguards had the suspicious vehicle towed away. Ten people were wounded when the car exploded shortly afterwards.
Guerrillas had fired a grenade at the mayor's house in 2003 and her father survived an assassination attempt two years later.
"We can't go running away. We are going to stay here and reinforce my security," Gonzalez said. "This is a risk we have to take."
Neiva, about 250 kilometres south of Bogota, and the surrounding areas are a corridor for drug traffic from neighbouring provinces that have been the focus of the US-funded operations. FARC's Teofilo Forero command, one of its more experienced fronts, is active in the area.