Nigeria's national police claimed victory Friday over a radical Islamist sect after its leader was killed by security forces but experts warned revenge attacks could occur and a leading human rights group demanded a probe into the killing.

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.

"This group operates under a charismatic leader. They will no more have any inspiration," national police spokesman Emmanuel Ojukwu told The Associated Press on Friday. "The leader who they thought was invincible and immortal has now been proved otherwise."

Ojukwu said there are still some isolated cases of violence in northern Nigeria, but otherwise "life is back to normal."

New York-based Human Rights Watch called for an investigation.

"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, Yusuf had discounted Darwin's theory of evolution, claimed the world cannot be round because the Quran does not say that and credited Allah with creating rain.

Witnesses said calm prevailed Friday morning in large sections of the northern city of Maiduguri, Yusuf's base and the capital of Borno state. But 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 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. Nigerian troops shelled the compound on Wednesday and the 39-year-old leader escaped with about 300 followers, some of them armed.

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

"I believe he was shot while he was trying to escape," the state governor's spokesman, Usman Ciroma, told AP.

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 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."

Yusuf's followers were estimated to rank in the thousands. Nnamdi K. Obasi, a Nigerian analyst with the International Crisis Group, said he 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.