Nine people accused of carrying out a deadly attack in north-central Nigeria were arraigned before a federal court on Monday, charged in connection with the deaths of more than 150 people.

The defendants are being charged with 57 counts of terrorism and could be sentenced to death or life imprisonment if convicted. They pleaded “not guilty” to the charges relating to the attack on Yelewata, a community in the Guma area of Nigeria’s Benue state, in June last year.

Presidential spokesperson Kamarudeen Ogundele said in a statement on Sunday that the Nigerian government undertook a “painstaking investigation and collaboration by government agencies.”

“The office of the Attorney General of the Federation and Minister of Justice wishes to assure Nigerians that Justice will be ensured in the matter to send a strong signal to the enemies of the country, acting under any disguise,” Ogundele said.

Nigeria is in the grip of a complex security crisis, with an insurgency by Islamic militants in the northeast alongside a surge in kidnappings for ransom by gunmen across the northwest and north-central regions over the recent months.

Attacks such as the one in Yelewata are common in Nigeria’s northern region, where local herders and farmers often clash over limited access to land and water. The prolonged conflict has become deadlier in recent years, with authorities and analysts warning that more herdsmen are taking up arms.

In December, the U.S. launched airstrikes in northern Nigeria, targeting fighters it claimed were from the Islamic State group, after allegations that the West African country failed to rein in attacks on Christians. Nigeria has recorded an uptick in attacks on churches in the north.

The U.S. has pledged to deliver military equipment purchased by Nigeria over the past five years but that has not yet arrived. The material, which includes drones, helicopters, platforms, spare parts and support systems, is intended to support the country’s efforts to rein in terror attacks.