Lagos, Nigeria (CNN) -- Authorities were searching for more than 680 inmates Wednesday who escaped after gunmen attacked a prison in northern Nigeria, police said.

The prison was attacked by suspected members of an Islamist sect called Boko Haram on Tuesday.

One police officer was killed and two prison officials were injured, police said.

The group freed 721 inmates, said Emmanuel Ojukwu, a police spokesman.

Police captured 35 of the escapees and were searching for the others, Ojukwu said.

About 200 members of the Boko Haram sect attacked the prison, triggering a massive gunbattle, according to authorities.

Police suspect the prison was attacked because it was holding 80 members of the sect.

Boko Haram has been at odds with Nigerian officials because the group wants the government to impose Islamic law, known as sharia, in the entire Muslim-dominated northern half of Nigeria.