ABUJA, Nigeria (AP) — At least seven members of a Nigerian government protection agency are missing after their convoy was attacked by Boko Haram insurgents in north-central Nigeria, authorities said.

A team of 80 security operatives tasked with protecting the power network were ambushed Monday in Shiroro, Niger state, by about 200 Boko Haram fighters during a patrol mission, Babawale Afolabi, a spokesperson for the Nigeria Security and Civil Defence Corps said late Tuesday. At least 50 insurgents were killed, Afolabi added.

Boko Haram is among armed groups known to sabotage power supplies in Nigeria. Last month, the northern part of the country was without power for more than seven days after insurgents damaged equipment, the state-owned Transmission Company of Nigeria said.

Although Boko Haram mainly operates in the northeast, the government says the group has cells in the largely Muslim Niger state, where its fighters ambushed the convoy and where they have previously carried attacks against the military and civilians.

The Nigerian insurgent group took up arms in 2009 to fight against Western education and impose their radical version of Islamic law, or Sharia. The conflict, which is now Africa’s longest struggle with militancy, has spilled into Nigeria’s northern neighbors.

Some 35,000 civilians have been killed and more than 2 million have been displaced in the northeastern region, according to the United Nations. The kidnapping in 2014 of 276 schoolgirls by Boko Haram from the village of Chibok in Borno state — the epicenter of the conflict — shocked the world.

In September, at least 100 villagers were killed in northeastern Nigeria when suspected Boko Haram fighters opened fire on a market, on worshippers and in people’s homes, according to residents.