At least 40 people were killed in an attack on a crowded market in Nigeria, an official told NBC News. State Sen. Ahmed Zanna said the attack took place in Nigeria's city of Maiduguru — which previously has been targeted for attack by Boko Haram militants.

Eyewitnesses told Reuters that a woman had entered the roadside trading area behind the city's main market before blowing herself up and that a second blast soon followed. "While the people were trying to help the injured the second bomb blasted," Sani Adamu told the news agency. "I saw lots of bodies."

Maiduguri, a city of some 600,000, is the capital of Borno state — a region which has experienced countless killings and kidnappings at the hands of Boko Haram. The group gained worldwide infamy in April when it kidnapped 276 schoolgirls, sparking the Twitter hashtag #BringBackOurGirls.

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- Alexander Smith

Reuters contributed to this report.