The United Nations says that a Chadian peacekeeper in northern Mali has killed two colleagues and injured another after shooting at them. The spokeswoman for the U.N. mission in Mali, Radhia Achouri, said Sunday that authorities have arrested the soldier who fired in the Saturday night shooting in Tessalit in Mali's northern Kidal region. She said the reason for the attack was not known and that investigations have been opened into the incident, AP reported.