A large car bomb has exploded in front of police headquarters in Colombia's south-west city of Cali, the first such attack in a major city for four years.
Tens of thousands of civilians have died in the long-running civil conflict
Officials said the blast, which killed one person and injured more than 30, was most likely the work of Farc, or Revolutionary Armed Forces of Colombia.
But drug traffickers, paramilitaries or common criminals have not been ruled out as suspects.
A $500,000 (£254,000) reward is being offered to catch those responsible.
Tens of thousands of civilians have been killed in Colombia's long-running civil conflict, which involves state forces, rebels and paramilitaries.
Both the guerrillas and paramilitaries are heavily involved in the drugs trade.
The car bomb, with an estimated 80kg of explosives, was parked outside a police barracks.
A passing taxi driver died in the attack and many of the wounded were police officers.
The BBC's Jeremy McDermott in Medellin says the authorities blame the Farc and if true, it shows the guerrillas have returned to the campaign of urban terrorism they abandoned four years ago.
The security policy of President Alvaro Uribe has managed to isolate the rebels from most of the major urban centres - yet the rebels have managed to keep a toehold in Cali.
The timing of the attack raises questions, with the second largest rebel group, the National Liberation Army (ELN), due to sit down this week with government officials to discuss a possible peace deal.
The Farc have accused the group of treason and are seeking to win over dissident ELN guerrillas that want to keep fighting - and a high-profile display of strength may be the way to do so, our correspondent says.