Leftist rebels in Colombia ambushed a group carrying regional election ballots, killing 12 members of the security forces, officials say.
The group had left an indigenous reservation in the northern Guican province when it was hit, defence minister Luis Carlos Villegas said.
He said the National Liberation Army (ELN), Colombia's second-biggest rebel group, was behind the attack.
President Juan Manuel Santos vowed retaliation against the attackers.
On Sunday, Mr Santos had said the election had been "the most peaceful and least violent in decades".
Eleven of those killed were soldiers, while the other was a police officer, Mr Villegas said.