Gunmen have stormed a prison in southwestern Mexico in an attack that left five of the assailants and four inmates dead, officials say.

Six armed men entered the prison in the Guerrero state town of Igualaon Friday by posing as public officials before firing at inmates and guards, the state prosecutor’s office said in a statement.

The gunmen fooled the guard at the prison entrance by telling him they were delivering an inmate sometime after midnight.

“Once in the prison, the armed group started a confrontation against inmates and later against guards in a security tower,” the prosecutor’s office said.

Two other people were injured in the attack, according to state police.

The motive behind the attack was not immediately known.

One of the dead inmates was a convicted cocaine dealer, another was a kidnapper and the two others were imprisoned for carrying illegal weapons.

The National Human Rights Commission said in November that violence has increased in Mexico’s prisons, the majority of which are controlled by inmates.