Rebel forces in Colombia attacked a group conveying regional ballots from Sunday’s election, killing a dozen security guards in the process.

The BBC reports, citing defense minister Luis Carlos Villegas, the ELN (Ejército de Liberación Nacional) attacked the group after they left an indigenous reservation located near the town of Guican.

Out of the dozen slain, 11 were soldiers and one was a police officer. Aside from the confirmed deaths, six others are still missing.

Through negotiations that have been taking place in Havana since 2012, peace has grown between the leftist rebel group FARC (Fuerzas Armadas Revolucionarias de Colombia) and the conservative Latin American government of Colombia, but the ELN has been left out of those talks.

Colombian leader Juan Manuel Santos has vowed retaliation against the ELN, which after FARC, is the second largest terrorist organization in the region.

The attack has marred an otherwise successful election, which Santos described on Sunday as "the most peaceful and least violent in decades."

Although negotiations between FARC and Colombia have been going well, back in May the leader of the rebel group, Timochenko, urged the government to include ELN in peace talks aimed at ending more than half a century of conflict.

Timochenko went so far as to call the matter urgent, as the BBC reports. 

"We believe that not only for us as a revolutionary movement is it urgent and necessary that the ELN is brought into the peace talks. It is also urgent for the government and for the Colombian people. It is the right and practical thing to do," he said.

In September, FARC and the Colombian government were able come to certain agreements regarding how they planned to punish rebels who had committed human rights abuses during the half century of conflict between them.