About 20 gunmen who bombed their way into a Nigerian prison and freed 119 prisoners Wednesday are thought to be members of Boko Haram, officials said. President Goodluck Jonathan\'s spokesman, Rueben Abati, said the gunmen stormed the prison in Koton Karifi, just south of the Nigerian capital Abuja, and killed a warden as they bombed open a gate, The Wall Street Journal reported. The Islamic extremist group hasn\'t claimed responsibility for the attack but is thought to be behind it, the Nigerian Tribune reported. If it does claims responsibility, the Journal said, it would mark the first major attack by the group in Nigeria\'s predominantly Christian south. The attack lasted 30 minutes, the Tribune said. Officials were attempting to determine details about the freed prisoners, Abati said. Boko Haram freed 60 prisoners in a Jan. 20 attack in the northern city of Kano in which at least 186 people died. The group also freed about 700 inmates in a 2010 attack in northern Bauchi state, the Journal said. Abati said in Wednesday\'s attack, militants may have been trying to free two of the group\'s leaders who were arrested recently, though neither was in the prison. \"If you know the mode of operation of Boko Haram, on previous occasions, they have attacked facilities where they thought their leaders were being held,\" Abati said. Human Rights Watch says Boko Haram, whose name means \"Western education is forbidden,\" has killed more than 935 people since 2009, while the Nigerian newspaper Vanguard says the group has killed more than 2,000 people in northern Nigeria.