Colonel Luis Eduardo Martinez, the regional police chief, said a suspect believed to belong to the Revolutionary Armed Forces of Colombia (Farc) group had been arrested.
If confirmed, it would mark the first such serious attack by the group in months.
It is not clear why the rebel group would target a local festival, and Colombian authorities often accuse the group even if it is not possible to verify who is behind the attack.
However, the group has a strong presence in Ituango, in the remote northern part of Antioquia state, where coca leaf is cultivated to manufacture cocaine for drug traffickers.
Alvaro Uribe, the Colombian president, was due to fly to the scene of the attack on Friday, local authorities said.
The Farc group, which has been fighting the Colombian government for decades, has recently been hit by the deaths of three senior commanders and a wave of desertions.
It also suffered a blow when its most prominent hostage, French Colombian politician Ingrid Betancourt, was freed from their custody along with several other hostages in a dramatic rescue by Colombian authorities in July.