POTISKUM, Nigeria (AP) — North Korean doctors hacked to death by machete-wielding attackers. Women vaccinating children against polio gunned down in the street. A top Islamic cleric, whose predecessors once served as ultimate rulers in the region, nearly killed in an ambush.

These recent attacks in northern Nigeria show the changing tactics of Islamic extremists here and continuing dangers facing Africa’s most populous nation, despite a buildup of soldiers and police officers, door-to-door searches by security forces and mass arrests. As the killings continue, analysts believe the fighters, likely part of the amorphous Islamic sect known as Boko Haram, slip easily in and out of Nigeria to launch attacks — putting other West African nations at risk.