Five Nigerian police officers have pleaded not guilty to charges that they killed the leader of a radical Islamic sect two years ago.

The officers were charged Tuesday at a court in the capital, Abuja, with “unlawfully killing” Mallam Mohammed Yusuf and some of his followers, and with committing a “terrorist act.”

Yusuf was the leader of the group Boko Haram, which launched an uprising across much of the north in 2009 that was put down by Nigeria's military in a campaign that killed more than 800 people, including Yusuf. The group has long charged that Yusuf died in police custody.

Boko Haram wants to impose strict Islamic law across the north, and it rejects Nigeria's constitution and federal government. The group was blamed for deadly bomb attacks and ambushes that caused an exodus earlier this month from Maiduguri in volatile Borno State.

Nigerian President Goodluck Jonathan has offered to hold formal talks with the sect to end the violence. But Boko Haram leaders say they cannot meet with security forces who are trying to destroy them.