At least 30 people including civilians and members of the military have died after an attempted rescue of an oil exploration team kidnapped by suspected Boko Haram militants in northeast Nigeria, Reuters reports.

The team searching for oil in Nigeria’s conflict-ridden northeast was made up of members from the state oil firm, NNPC and university researchers and staff, while the attempted rescue was carried out by military personnel and vigilantes. The NNPC, which contracted university staff, has for more than a year surveyed what it says may be vast oil reserves in the Lake Chad Basin.

Nigeria’s government and military have repeatedly said Boko Haram is on the verge of defeat. In December, President Muhammadu Buhari said the group’s last stronghold, an enclave in the Sambisa forest, had fallen. But insurgents have launched attacks with renewed zeal in the past few months. The latest attack brought the death toll in Maiduguri and its environs since early June to at least 92.