(CNN) -- Gunmen stormed a school in Nigeria killing 20 students and a teacher over the weekend, state news reported. Four more students suffered critical wounds, when the men opened fire early Saturday, Voice of Nigeria said. An investigation is underway to find the suspects. No group immediately claimed responsibility for the incident. The attack occurred in the state of Yobe, in Nigeria's northeast, where Islamist extremists from the terror group Boko Haram regularly carry out attacks. An attack on a school in June in the same state took the lives of seven students and two teachers, VoN reported. Boko Haram claimed responsibility for that attack, according to local media reports. Violence flares in tense Nigerian region .
The gunmen also critically wounded four more students . The region is known for attacks by Islamist terrorists from Boko Haram . Seven students died in a similar attack in June in the same state .