The teenage girls entered the busy marketplace separately Tuesday, their vests of explosives hidden beneath their full hijabs.

The first detonated her bomb, killing an unknown number. As rescuers rushed in, the second girl screamed and set off her explosives, killing dozens more, according to witnesses and authorities.

More than 40 people died in the double suicide bombing in Maiduguri, the capital of Borno state in northeastern Nigeria, according to Haruna Issa, a hospital volunteer in the city.

Suspicion immediately fell on the insurgents from the Islamic extremist group Boko Haram, which controls a large part of northeastern Nigeria and is blamed for the deaths this year of at least 1,500 people in Africa’s most populous country.

In its campaign of violence, Boko Haram has used car bombs and men wearing explosive vests. It also has begun using women who can cover the explosives with their hijabs, and the recruits appear to have gotten younger, with several instances of teenage attackers this year.

The militants attracted international attention with their April kidnapping of more than 200 schoolgirls from Chibok, about 78 miles southwest of Maiduguri. The schoolgirls are still missing and their plight has aroused international concern and prompted the social media campaign, #BringBackOurGirls.

On Oct. 17, the parents of the schoolgirls were encouraged when the Nigerian military announced a cease-fire with Boko Haram and said negotiations had begun for the release of the captives.

Those hopes were quickly dashed when Boko Haram fighters continued attacks and seized several cities and towns across the northeast. In a video statement, Boko Haram leader Abubakar Shekau denied the cease-fire and scoffed at claims of negotiations to release the schoolgirls.

It was not known whether Tuesday’s attackers were connected with the April abduction. A worker with a non-governmental organization said young women in northern Nigeria are especially vulnerable to recruitment by the extremist group. The worker spoke on the condition of anonymity because she feared for her safety.

The bombings by the two girls took place as the marketplace was crowded with shoppers, said Abba Aji Kalli, coordinator of the Civilian Joint Task Force in Borno state.

The first girl set off her explosives, while the second apparently waited until the rescuers rushed in to help before detonating her bomb, killing dozens of others, Kalli said.