A hand grenade attack killed two students and injured six in a university in southern Ethiopia, police said, the second such attack in the country in three weeks.

An attacker or attackers hurled the grenade at students on the campus at Dilla University in the diverse Southern Nations, Nationalities, and Peoples Region (SNNPR) on Thursday evening, officers added. They did not give any details on any reasons for the attack in an area, about 350 km (200 miles) south of the capital Addis Ababa, where there have not been any recent reports of significant unrest.

“The blast killed two students and wounded six, of which four were seriously injured,” a police official in the town, Ejigu Shiferaw, told the state-run Ethiopian News Agency. The outlet said an unspecified number of suspects were in police custody, without giving further details.

The attack took place 21 days after a bomb was thrown at worshippers in a mosque in Addis Ababa, injuring more than 20 people. Officials are yet to disclose details of that attack, and there has not been any claim of responsibility.

Ethiopia says it has thwarted several bomb plots in recent years and blames rebel groups based in other areas of the south and southeast, as well as al Shabaab insurgents from neighboring Somalia.

Ethiopian troops have been fighting al Qaeda-allied al Shabaab militants in Somalia since 2011 as part of an African Union peacekeeping mission.