Suicide bombers attacked a checkpoint in Nigeria’s northeastern city of Maiduguri, killing two of themselves before dawn Friday. Two other explosions occurred hours later.

It was the fifth attack in three weeks on the city that is the birthplace of Nigeria’s homegrown Islamic extremist group, Boko Haram.

Before 4 a.m. Friday, police challenged two women and a man running toward the checkpoint opposite the Federal High Court, deputy police superintendent Victor Isuku said. One woman detonated, killing herself and the male accomplice. The other woman has been arrested and is being interrogated, he said.

No details were immediately available about two blasts that rang out around 8 a.m. (0700 GMT).

Boko Haram has recently stepped up attacks after a months-long lull in their Islamic uprising that has killed more than 20,000 people over seven years.

Security forces have managed to foil most of the attacks.

On Oct. 29, two women suicide bombers blew up outside a refugee camp, killing five people. The next day, troops shot and killed a man with explosives strapped to his torso as he ran toward the same camp. On Nov. 1, a car bomb targeting a military checkpoint leading to Maiduguri exploded, killing all nine people in the vehicle. Last week, soldiers shot at three female suicide bombers, detonating some of their explosives and killing all three.

Nigerian troops this year have driven the insurgents out of most towns in the northeast. But now the insurgents are fleeing south into the central-eastern state of Taraba, the state director for Nigeria’s secret intelligence agency, Shehu Saulawa, told senators there on Thursday.

The insurgency has spread across Nigeria’s borders, forced some 2.6 million people, mainly farmers, from their homes and created a massive humanitarian crisis in which the United Nations says 14 million people face starvation.