The immediate crisis may be over in Nigeria, but the threat of violence remains. Government security forces today attacked a mosque filled with Islamist militants, killing scores of fighters and forcing more to flee.

The militants, blamed for days of violence across the country’s north, belong to a group known as Boko Haram, which aims to overthrow the federal government in Abuja and impose a strict version of Islamic law.

The sect’s leader Mohammed Yusuf escaped the raid along with some 300 of his men, but was later arrested and then died in custody according to police. Four days of clashes, sparked by attacks on police stations and government buildings, have killed at least 300 people.

Also known as Nigeria’s Taliban, Boko Haram formed about eight years ago. A huge government operation against the group in 2004 ended with the police claiming victory.

But five years on and the militants are back, stronger and more vicious. In the latest outbreak of violence, in Maiduguri, the capital of northeastern Borno state, militant gunmen assaulted police stations and engaged armored-personnel backed troops Africa’s most populous country sits on a religious fault line.

Its 150 million people are split almost evenly between Muslims in the north and Christians in the south. For many years, the northern Muslim élite have dominated Nigerian politics, using their positions to enrich themselves and their families.

"We have seen this country degenerate from a promising state to a dysfunctional one. We have seen unmitigated corruption and insensitivity on the part of its rulers,” says Mohammed Ndume, a federal MP from Borno state.

"We are seeing a lack of opportunities and so much stress for its people. So what do you expect? The people have to react, and that is what you are seeing and that is what the current crisis is all about.”

Over the past few years a new breed of young Muslim activists, most of them educated and from the middle class, have aggressively embraced a stricter version of Islam, rejecting anything Western and Christian.

Boko Haram began life as a peaceful group focused on the study of the Koran, according to Abdulmumin Sa’ad, a Muslim scholar and professor of sociology at the University of Maiduguri.

"The idea was that there is a lot of sin in the larger society and their parents had amassed a lot of ill-gotten wealth,” says Sa’ad, who taught some of the militants.

"There is widespread immorality, and so the best thing to do is to remove themselves and camp elsewhere, where they can concentrate on their religion ... mediate, reach out and begin to form a fellowship.” Sa’ad claims that the group turned violent when authorities harassed it.

The Islamist group is not the only threat facing Nigeria. The country is one of Africa’s biggest oil exporters, and yet some 70% of its people live in abject poverty.

A string of devastating attacks by militants demanding a greater share of the oil wealth in the Niger Delta, in the south, has reduced oil output by a third, hitting government revenues. This week’s fighting will add to the sense that the government is losing control.

"The government is no longer in control of the security situation outside the main cities,” says a senior U.S. diplomat in Abuja. "You can’t drive in the countryside at night and not get attacked, and sometimes in the daytime as well.”

Western diplomats worry that the security vacuum may allow foreign terrorist groups such as al-Qaeda to move in. Osama bin Laden is widely admired in the arid north. It has become fashionable for Muslims to name their sons after him, while his picture adorns T-shirts and posters.

In a speech in 2000, bin Laden named Nigeria as among "the region[s] most qualified for liberation.” "Clearly there is a lot of concern in Washington with the idea that al-Qaeda can gain a foothold within the 65 million–strong Muslim population in northern Nigeria,” says the U.S. official. Even if that doesn’t happen, local extremist groups could present a headache for years to come.

TIME