The kidnap of hundreds of women and children by Boko Haram shows that the Nigerian militant group will continue to survive despite the successes of the Nigerian military’s counter-insurgency campaign in the country’s northeastern regions, a former U.S. ambassador to Nigeria has said.

As the terror group retreated from the town of Damasak in Borno state earlier this month it kidnapped as many as 500 children and women, according to local reports. Nigerian military spokesman Mike Omeri confirmed the large-scale abduction in the town but could not specify the number of people kidnapped, in comments made to the Associated Press. He added that the extremists rounded up students and teachers in Damasak’s schools before retreating from the town.

John Campbell, former U.S. ambassador to Nigeria and senior fellow for Africa policy studies at the Council on Foreign Relations (CFR), says the kidnap demonstrates that, despite the gains made by the Nigerian military in its six-week offensive against Boko Haram ahead of Saturday’s presidential election, the group will survive by going underground.