MAIDUGURI, Nigeria - Police in northern Nigeria are investigating how a homegrown Islamist sect managed to unleash a wave of violence that left more than 700 people dead last week, a police spokesman said Monday.

Moderate Muslim clerics and scholars said they had warned government officials about the sect's violent tendencies -- and that the alarms went unheeded before Boko Haram militants attacked a police station in Bauchi state on July 26.

Violence quickly spread to three other states before Nigerian forces retaliated, storming the group's Maiduguri compound Wednesday and killing more than 100 fighters.

Police in the past few days have arrested 20 suspected members of the sect, Borno state police spokesman Isa Azaza said. Some were found with stocks of gunpowder, vats of acid, aluminum containers for bomb-making, assault rifles and black-and-white flags with Arabic inscriptions, Azaza said.

He declined to say where the men were from. Many Boko Haram members were purportedly from neighbouring Niger.

"Investigations are ongoing," he said, adding that police were still searching for fugitives.

Azaza said the group was well organized in its attacks on police stations and government installations.

"They divided themselves and attacked all the police stations in the metropolitan area," he said. "It was a well-planned attack."

The city of Maiduguri was calm Monday morning, with police conducting street patrols in armoured vehicles that were bristling with armed policemen. Police also guarded key intersections and searched passers-by.

Army Col. Ben Ahanotu said many sect members are still at large.

"There are lots of them still around ... only a few of them are still dangerous," he told The Associated Press.

Suspicions that Mohammed Yusuf, the sect leader, had been executed after being captured Thursday grew deeper after a photo emerged Sunday on news Web sites showing him in custody of soldiers with only a wound to an arm. The military said it handed him over alive to police, while police insist he was killed in a gunbattle.

Boko Haram is also known as the Nigerian Taliban. No direct link to al-Qaida has emerged but the bloodshed comes amid mounting concern about al-Qaida affiliates' ability to cross desert borders of North Africa.

Boko Haram -- translated as "Western education is sacrilege" -- seeks the imposition of strict Islamic Shariah law in Nigeria, a multi-religious country that is a major oil producer and Africa's most populous nation.

The country's most prominent opposition group has criticized security services for not tackling what it described as a key recruiting tool for groups like Boko Haram: the abject poverty of the north.