An international human rights group called Friday for an immediate investigation into the fatal shooting in police custody of a radical Islamist leader blamed for instigating days of clashes between his followers and government forces.

Nigerian officials said Mohammed Yusuf, the leader of the sect some call the Nigerian Taliban, was killed after being captured Thursday night at the end of a four-day manhunt.

Witnesses said calm prevailed Friday morning in large sections of the northern city of Maidugiri, Yusuf's base and the capital of Borno state, but those reports could not immediately be confirmed. Experts said it was not clear yet if Yusuf's death would end the violence in northern Nigeria or inspire revenge attacks by the Boko Haram sect, which seeks the imposition of strict Islamic Shariah law in the multi-religious country.

Yusuf, a Western-educated member of the country's elite, had encouraged his followers to rid themselves of all material wealth while he was chauffeured around in a Mercedes all-terrain vehicle and amassed dozens of vehicles at his compound.

Officials imposed partial Shariah in much of the north but Boko Haram members were increasingly angry that full Islamic law had not been implemented, especially the law's demand for a social welfare system helping poor people. The militants attacked police stations, churches, prisons and government buildings in a wave of violence that began Sunday in Borno and quickly spread to three other northern states.

President Umaru Yar'Adua said that security agents had been ordered to attack when the movement started gathering fighters from nearby states at its sprawling Maiduguri compound in preparation for "the holy war."

Nigerian troops shelled the compound on Wednesday and the 39-year-old leader managed to escape with about 300 followers, some of them armed.

Officials said he was found Thursday in a goat pen at his in-laws' home in the northern town of Kernawa.

The state governor's spokesman, Usman Ciroma, told The Associated Press Thursday night that: "I saw his body at police headquarters," in Maiduguri.

"I believe he was shot while he was trying to escape," Ciroma said.

New York-based Human Rights Watch called the reports of Yusuf's death "extremely worrying."

"The Nigerian authorities must act immediately to investigate and hold to account all those responsible for this unlawful killing and any others associated with the recent violence in northern Nigeria," said Corinne Dufka, the group's senior West Africa researcher.

A university graduate who spoke excellent English, Yusuf had discounted Darwinism, claimed the world cannot be round because the Koran does not say that and credited Allah with creating rain.

Security officials said the leader had four wives. Their whereabouts are unknown.

His followers were estimated to rank in the thousands. Nnamdi K. Obasi, a Nigerian analyst with the International Crisis Group, said Yusuf seemed to have several hundred core followers in the capitals of 12 northern states and a few thousand supporters in each urban center. The sect is strongest in the northeast states of Borno, Bauchi and Yobe.

Yusuf's followers believe the greatest honor is to die fighting for their cause, Obasi said.

Leading Nigerian rights groups accuse security forces of killing bystanders and other civilians before and during the Wednesday siege of Boko Haram's compound. A military spokesman denied the charge and said it was impossible for rights workers to tell who was a civilian and who was a member of Boko Haram, which means "Western education is sin" in the local Hausa language.

Troops killed about 100 militants by an AP reporter's count, half of them inside the sect's mosque, and the bodies of barefoot young men littered the streets of Maiduguri on Thursday morning as the army pursued the manhunt on the outskirts of the city.

The militants are also known as Al-Sunna wal Jamma, or "Followers of Mohammed's Teachings," and some Nigerian officials have referred to them as Taliban. Obasi said a few have fought with that radical movement in Afghanistan.

Nigeria's 140 million people are roughly divided between Christians in the south and northern-based Muslims. Shariah was implemented in 12 northern states after Nigeria returned to civilian rule in 1999 following years of oppressive military regimes. More than 10,000 Nigerians have died in sectarian violence since then.

Dire poverty is at the heart of the violence, which analysts say reflects decades-old grievances of Nigerians whose governments are so corrupt and ineffective they do not deliver even basic services like running water and electricity.