Continental Gold exploration geologists and contractors resting at a residence in Colombia were the victims of an “armed attack” in the middle of the night, the company reported in a press release Thursday.

The incident took place in the village of Ochalí, in the municipality of Yaramul, Antioquia, within the boundaries of Continental’s Berlin project.

The company said the details and the severity of the attack are unclear, but that “the worst is feared.” Continental said it is cooperating with the local authorities and working with families affected by the attack.

Related: Continental Gold aims to bring its Buriticá project to production in two years – an important milestone for the company and for Colombia

The company said it believes the attack was perpetrated by former members of the Revolutionary Armed Forces of Colombia (FARC), a political dissident group that was active in the country from 1964 to 2017 and were involved in drug trafficking and kidnapping.

In June 2017, the group officially dearmed themselves, handing over their weapons to the United Nations as part of a historic peace deal, before reforming as a political party a month later. Several dissident groups have since continued the group’s original activities.  

Local newspaper El Colombiano reported that the assailants arrived on the site while the workers were sleeping and fired on them. As a result, three workers — identified as Henry Mauricio Martínez Gómez, Laura Alejandra Flórez Aguirre and Camilo Andrés Tirado Farak — were killed. Another three workers were injured in the attack, and five were left unharmed.

This is the second attack at a Continental Gold site this month. On Sept. 5, two engineers working for Continental near the company’s Buriticá project were attacked by two armed men. One of the engineers, Oscar Alarcon, was killed while the other was injured. The company said the attack on Thursday was not related to the Buriticá attack, as the two sites are eight hours away from each other by car.