The bloodshed sparked fears of renewed unrest in the area, which had been relatively calm for the last several years.
Col. Abdoul Ndiaye said late Saturday that seven others were wounded in the massacre 4 miles (7 kilometers) outside of the town of Ziguinchor, and the military stepped up its presence near the town. Casamance is separated from the rest of Senegal by the nation of Gambia.
While no immediate claim of responsibility for the slayings was made, suspicion fell on the separatist group founded in 1982 known as the Movement of Democratic Forces of Casamance.
The armed wing of the group had agreed to a cease-fire in 2014, and the last major attack blamed on the group had been in 2013 when rebels took 12 employees of a South African bomb disposal firm hostage. The victims, all Senegalese citizens, were eventually released.
Saturday's deaths came just hours after the release of two prisoners belonging to the separatist group following negotiations that were mediated by the Community of Sant'Egidio.
The separatists have long argued that their region is culturally distinct from the rest of Senegal, and has suffered from inattentive governments in the country's capital, Dakar.
Associated Press writer Mamadou Alpha Diallo in Ziguinchor, Senegal, contributed to this report.
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