MAIDUGURI, Nigeria (AP) — Armed extremists in northeastern Nigeria killed dozens of people earlier this week in separate attacks targeting a construction site and a military installation, security officials said Saturday.

Gunmen killed at least 25 construction workers during an ambush on Thursday in the town of Sabon Gari in Borno State, a senior officer of the Borno State Police Command told The Associated Press. He spoke on condition of anonymity because he was not authorized to speak to reporters. Authorities in Nigeria often decline to publicly confirm death tolls in attacks, citing security concerns.

“It is a devastating loss, and the hallmarks point directly to Boko Haram insurgents who have long resisted developmental projects in these areas,” the police official said.

Boko Haram, Nigeria’s homegrown jihadis, took up arms in 2009 to fight Western education and impose their radical version of Islamic law.

The insurgency now includes an offshoot of the Islamic State group known as the Islamic State West Africa Province, or ISWAP. It has spilled into Nigeria’s northern neighbors, including Niger, killing about 35,000 civilians and displacing more than 2 million people, according to the United Nations.

Abdurrahman Buni, a senior officer of the Civilian Joint Task Force, a volunteer vigilante group helping the military fight extremist groups and armed gangs, confirmed that at least 25 construction workers were killed during the Thursday attack.

Buni and the police officer said extremist fighters, backed by armed drones, had raided an army base in a separate attack in the same town hours earlier. The police officer said the dead were nine soldiers and two members of a civilian task force, while about 16 injured security personnel were evacuated for medical treatment following the heavy gunfire.

He said it was unclear if the base attack was carried out by Boko Haram or the rival ISWAP, both of which are active in the region.

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,

Last month, the U.S. launched airstrikes in northern Nigeria, targeting Islamic State fighters, after allegations that the West African country failed to rein in attacks on Christians.