Human-rights activists have voiced concern over the death of the leader of an Islamic sect in Nigerian police custody, calling it "unlawful" killing.
Nigerian government officials said Mohammed Yusuf, 39, was shot while trying to escape. His capture by police had been announced just hours earlier.
His group is blamed for days of unrest that has left hundreds of people dead.
The Boko Haram group wants to overthrow the Nigerian government and impose a strict version of Islamic law.
Mr Yusuf was held and later shot in the north-eastern city of Maiduguri.
A BBC reporter in the city was among journalists shown two films, one apparently showing Mr Yusuf making a confession; the other showing what appeared to be his body, riddled with bullets.
"Mohammed Yusuf was killed by security forces in a shootout while trying to escape," the regional police assistant inspector-general, Moses Anegbode, told Nigerian television.
A spokesman for the state governor was also quoted as saying that Mr Yusuf had been trying to escape.
Troops had stormed Boko Haram’s stronghold on Wednesday night, killing many of the militants and forcing others to flee.
Mr Yusuf was arrested earlier on Thursday, after reportedly being found hiding in a goat pen at his parents-in-law’s house.
‘Gun battles’
Staff at Human Rights Watch said there should be an immediate investigation into the case.
"The extrajudicial killing of Mr Yusuf in police custody is a shocking example of the brazen contempt by the Nigerian police for the rule of law," said Human Rights Watch’s Eric Guttschuss.
Another Human Rights Watch researcher, Corinne Dufka, told AP news agency: "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."
The violence began on Sunday night in Bauchi state, before spreading to other towns and cities in the northeast of the West African nation.
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Crowds of militants tried to storm government buildings and the city’s police headquarters, but dozens of them were shot dead by security forces.
Several days of gun battles between militants and Nigerian security forces ensued, culminating in the assault on the militant’s stronghold.
It is thought more than 300 people have died in the violence – some estimates say 600, although there has been no official confirmation.
The Red Cross said about 3,500 people had fled the fighting and were being housed in their camp.
Witnesses and human rights groups have accused the military of excessive violence in quelling the militants, but the army says it used a minimal amount of force.
Police say Mr Yusuf was a preacher from Yobe state, who had four wives and 12 children.
They described him as a inspirational character.
His sect, Boko Haram, is against Western education. It believes Nigeria’s government is being corrupted by Western ideas and wants to see Islamic law imposed across Nigeria.
Sharia law is in place across northern Nigeria, but there is no history of al-Qaeda-linked violence.
The country’s 150 million people are split almost equally between Muslims in the north and Christians in the south.
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