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MAIDUGURI, Nigeria — A rash of mysterious killings by gun-wielding motorcycle assassins of policemen, politicians and others in this city near the desert has led authorities to declare that a radical Islamic sect thought to have been crushed by Nigerian troops last year has been revived.

Soldiers have been deployed here again, a curfew has been imposed and many residents worry about bold daylight attacks that officials call a renewal of the anti-Western sect’s strikes on police stations and soldiers that took place last year.

An outright challenge to the Nigerian government appears to be under way, with an audacious twilight prison break last month in Bauchi that freed over 700 — including many jailed sect members — the firebombing of a police station in Maiduguri last week and the killing of numerous police officers and other leaders in recent months.

The violence here in the north comes at a delicate time for Nigeria, one of the world’s top oil producers and a major supplier to the United States. Though the nation remains stable, it is struggling to organize elections next year that will test the capacity and, ultimately, the legitimacy of its young democracy.

Beyond that, the government faces a renewed threat from militants in the oil-producing south, who claimed responsibility for a deadly bombing during Independence Day celebrations in the capital, Abuja, this month. The southern militants had been waging an insurgency for years against the oil industry, but the bombing was the first time they had struck so directly at the heart of Nigerian power.

There, as here, the restiveness is fueled by corruption and glaring economic inequality. States in this region are the country’s poorest, with over 70 percent of the population living in poverty, according to the United Nations, while the few rich live in mansions behind high walls.

In Maiduguri, a hot, low-rise city of about one million people near the border with Cameroon — where exhortations to Allah are posted at traffic circles, women are veiled and bands of ragged boys carry plastic begging bowls — the discontent is tinged with religion. Islamic law is in force here, as it is across Nigeria’s north, but not strictly enough for the sect, Boko Haram, whose name is an expression in the local language, Hausa, indicating disgust with Western education.

In the market, men in flowing robes expressed anger at the government, which violently suppressed Boko Haram in a military operation last year that killed perhaps 800 people, but not at the sect members suspected of the recent killings.

“It’s the government’s fault,” said Alhaji Abdullahi Malari, a cloth trader. “Our representatives and our government, they are not sincere. What one person acquires is enough to care for a massive amount of people.”

A twine merchant, Alhaji Abu Abaja, concurred: “If government money was equally shared, there would not be this problem.”

Last year’s bloodshed and destruction is visible in buildings that remain riddled with bullet holes and burned-out vehicles that have still not been cleared. It did little to address the underlying social problems that led to unrest here, just as it apparently failed to stop the sect’s campaign.

“It’s an actual insurrection,” said Paul Lubeck, a specialist on northern Nigeria at the University of California, Santa Cruz. He added: “The attack on the Bauchi prison is much more sophisticated than anything that’s been seen. They’re drawing, for cannon fire, on massive poverty.”

The police here say 11 people have been gunned down in Maiduguri since July — the local news media count 14, including 6 policemen — in a series of killings by stocky, taciturn men on motorcycles wielding Kalashnikov rifles, who shoot their victims and drive off, sometimes firing into the air for good measure.

Two more policemen were killed in Bauchi late Thursday night, prompting a nighttime motorcycle ban there, along with the one in force in Maiduguri.

The police, as symbols of hated government authority, are particular targets, as they have been during religious unrest in the past. But some killings have aroused particular fear because of the widening circle of victims.

The national vice chairman of the All Nigeria People’s Party, Alhaji Awana Ali Ngala, was killed in his living room on Oct. 6. Three days later, Sheik Bashir Mustapha, a prominent Islamic cleric critical of Boko Haram, was killed while teaching in his home.

Bullet holes pockmark the walls, still smeared with blood, where the cleric was shot. The gunmen had greeted a 13-year-old student of the cleric on a quiet Saturday afternoon, asking him to show them inside. The boy then watched in horror as they gunned down his teacher and another student.

Yet out of fear or neglect, the police still had not interviewed the witness or even collected the many bullets left behind, angering students and families.

“He would always say, ‘Killing people is not religious,’ ” a student remembered. “He would go to the radio station and say that.”

About 250 miles away, on the muddy streets behind the Bauchi prison, through which hundreds of freed Boko Haram members streamed after their liberation last month by machine-gun-wielding sect members shouting “Allahu akbar!” there was sympathy for the sect. “Boko Haram is fighting the government because of the level of injustice,” said Dan Lami Aminu, a mechanic. “All the people are in support.”

The jubilant former prisoners marched past Mohammadu Bello’s tire repair shack that evening. “What they did was right,” Mr. Bello said. “There is serious injustice.”

The prison attack, on Sept. 8, took place at the busiest intersection in town, while crowds moved between the pink-walled Emir’s Palace and the sprawling central mosque, full of worshipers for the end of Ramadan. Amazed, people watched through plate-glass windows of the mosque as Boko Haram members calmly blasted into the central prison.

Still, the wave of recent killings has fed apprehension. The nighttime streets in Maiduguri, controlled by soldiers at numerous checkpoints, are deserted. A list of prominent targets — opponents of the sect — is believed to be circulating.

“There is fear,” said Ibrahim Abuya, chief imam of the main mosque in Maiduguri. “People are openly attacked in daylight.”

A sociologist at the university here, Abdul Mumin-Sa’ad, spoke reluctantly: “You don’t know when it’s going to be next, or who. It’s a very scary situation.”

Dozens of Boko Haram members are still being prosecuted in the wake of last year’s attack. The mysterious death of their leader, Mohammed Yusuf, while in police custody remains unresolved. Memories of bodies piled up from last year are fresh.

“They were summarily executed; the authorities didn’t even listen to them,” said Abba Babangida, a merchant, across the street from the charred police station that was firebombed last week. “They just killed them. So people have to sympathize.”