MAIDUGURI, Nigeria — Two suicide bombers, at least one of them a woman, blew themselves up on Tuesday at a crowded market in this northeast Nigerian city, killing dozens of shoppers and merchants, including some who witnesses said were decapitated by the explosions.

Suspicion immediately fell on Boko Haram, the Islamist insurgent group that was once headquartered here and has increasingly used female suicide bombers in recent months. Whether they are volunteers or are coerced is unclear.

It was the first time in more than four months that Maiduguri, the capital of Borno State, had been assaulted by suicide attackers. They targeted the same location, known as Monday Market, that was ravaged by a suspected member of Boko Haram in a car bombing in July.

“It is a serious crime against humanity being perpetrated by few extremists under the guise of religion,” said Kashim Shettima, the Borno governor, who visited some of the wounded victims at State Specialists Hospital.