A group of nine people conducting environmental research in northern Colombia were reported missing Sunday, three days after they were supposed to return from their expedition.

"We are calling them missing because we have received no word from an illegal armed group that tells us they've been kidnapped," said Virgilio Calderon, director of Corpocesar, the environment office in the local government of the province of Cesar.

He said the all-Colombian team was made up of five researchers from Corpocesar, a cameraman, two guides and a driver. The group has cooperation agreements with international environmental agencies, but all those missing were hired by Corpocesar.

Calderon said the group left Tuesday for the Sierra de Perija, some 600 kilometers north of Bogota, and were to return Thursday.

The province of Cesar is home to leftist rebels and far-right paramilitaries locked in a four-decade civil war.

Colombia has one of the highest kidnapping rates in the world.