Colombia threatened Tuesday to denounce Cuba at the United Nations unless it immediately turns over two guerrilla commanders believed to be living on the communist island.

The demand came in a letter sent to the Cuban Embassy in Bogota.

President Ivan Duque first pressed Cuba to arrest two National Liberation Army leaders after the group claimed responsibility for a car bombing in January that killed 22 people at a Bogota police academy.

But now the Duque government warns that it could denounce Cuba as a state sponsor of terrorism at the upcoming U.N. General Assembly because of the threat of an alliance between the ELN and a rearmed faction of the Revolutionary Armed Forces of Colombia, or FARC.

Read more: Washington Post