At least four people were killed and 14 others injured in a confusing attack on protesting coca farmers in a rural area of Tumaco city of Colombia on Thursday.
The authorities blamed on the dissident faction of a disarmed guerrilla group and local activists attributed to government security forces opening fire.
The disputed incident underscores the security challenges Colombia faces as it tries to uproot a booming harvest for coca, the raw ingredient for cocaine.
The market is a powerful magnet for dissident rebels and criminal gangs even after last year's deal to end a half-century conflict with the Revolutionary Armed Forces of Colombia, reports AP.
The now-disarmed guerrilla group commonly known by the acronym FARC was long a dominant player in the narcotics trade and used it to fund its insurgency.
Colombia's Defense Ministry said holdout rebels launched five homemade mortars and fired weapons at coca growers who were demonstrating in the area that produces the largest amount of cocaine coming out of the South American nation.
Clashes between coca growers and security forces have become more frequent as production of cocaine in Colombia surged to record levels in recent years.