Carlos Holguin, Colombia's interior minister, told local radio: "Police urged them against going to that area.
"There are still investigations under way, but we believe this is the Farc [Revolutionary Armed Forces of Colombia]."
The Marxist-inspired Farc has been involved in a four-decade war with Colombia's government and right-wing paramilitaries, many of which have now been disbanded.
Thousands of former paramilitaries have, however, joined criminal gangs which have previously carried out threats, kidnappings and assassinations to intimidate officials into cutting them in on public contracts.
Alvaro Uribe, Colombia's president, announced on Saturday a $25,000 reward for any information leading to the arrest of the "terrorists" behind the murders of candidates in the October 28 elections.
|"This year it is easier for |
them to kill candidates than to try to ban elections outright. Paradoxically, it is a sign of progress"
Claudia Lopez, Electoral Observation Mission
Two people were killed and at least six others injured when a grenade was thrown into a candidate's campaign headquarters in Puerto Asis in Putumayo province near the Ecuadorean border on Monday.
The candidate's sister was one of those killed, police said.
The conflict with the Farc has eased under Uribe, who has received widespread support for his US-backed campaign to improve security and fight drug trafficking.
But killings, threats and attacks have surged in the build-up to the vote.
However, Claudia Lopez, a member of Colombia's independent Electoral Observation Mission, said on Friday that the violence was actually a sign of progress since the last local elections in 2003.
"In 2003, the guerrillas had more territorial control and in some of those areas they did not allow elections at all," she said.
"This year it is easier for them to kill candidates than to try to ban elections outright. Paradoxically, it is a sign of progress."