Malian military spokesman Modibo Traore says Yoro Ould Daha, who was with the group MUJAO that controlled Gao for nearly a year, was arrested this week at his home.
Ould Daha was the MUJAO official who announced the death of French hostage Gilberto Rodriguez-Leal, who was captured in November 2012 while travelling in Mauritania and Mali.
He also took responsibility for the kidnapping of five humanitarian workers who were later released.
MUJAO, or the Movement for Oneness and Jihad in West Africa, controlled Gao when Mali's northern half fell to Islamic extremists after a military coup in March 2012.