DAKAR, Senegal — The human rights group Amnesty International said Monday that it had satellite images suggesting mass graves in the northeastern Nigerian city of Maiduguri, following what witnesses have described as the large-scale execution of inmates by security forces at a military prison on March 14. The group said the pictures showed shapes often seen in satellite images of mass graves. Its report quotes witnesses who said they observed soldiers conducting mass shootings of frail detainees. The shootings took place after an attempted prison break by the Islamist group Boko Haram. Over 600 were killed that day by security forces, the rights group said, making it what was most likely the deadliest episode in Boko Haram’s five-year insurgency. Amnesty suggested that both the Islamists and the Nigerian Army were guilty of crimes against humanity.