Garissa , Kenya ( CNN ) The desks of the small Madrassa are empty . Its 573 students , all male , are staying home after Kenyan President Uhuru Kenyatta announced three days of national mourning following last week 's deadly attack at a nearby university . Only a few kilometers away , 147 people -- mostly students -- were brutally massacred when Al-Shabaab militants invaded the campus in Garissa , a town in northeastern Kenya . We 've come to this particular Islamic religious school because the man suspected by Kenyan authorities of being the `` mastermind '' behind the atrocity -- Mohamed Mohamud -- once taught here . `` He was someone who was very quiet , he did n't like too much talk , '' recalls Sheikh Khalif Abdi Hussein , the principal at the Madrassa . He says he also taught with Mohamud for two years . `` When he left the Madrassa , he joined Al-Shabaab . But before , he was normal , just like me and other people . '' What worries authorities here is exactly that -- Mohamud was Kenyan . But now , say officials , Mohamud is in command of an Al-Shabaab militia based near Kenya 's long , porous border with Somalia -- about 118 miles ( 190km ) from Garissa -- who are believed to be responsible for numerous cross-border attacks into Kenya . The Islamist militant group , who are allied with al Qaeda , have been waging a bloody campaign for control of Somalia . With Kenyan troops part of an African Union force deployed in support of Somalia 's United Nations-supported government , Kenya has now become a target . Last year , an attack by Al-Shabaab on a shopping center in the country 's capital , Nairobi , claimed the lives of 68 people . Now Mohamud stands accused of being behind Thursday 's attack -- the deadliest attack in the nation since al Qaeda killed more than 200 people at the U.S. Embassy in Nairobi in 1998 . But Mohamud is not Kenya 's only homegrown terrorist . The Kenyan Interior Ministry has said at least one of the four gunmen who carried out the attack on the university was also Kenyan . Abdirahim Abdullahi was in his 20s and the son of a government chief . His father says he lost contact with his son in 2013 , shortly after he left university . The Kenyan government is concerned that Al-Shabaab is recruiting disaffected youth from inside the country . `` Our task of countering terrorism has been made all the more difficult by the fact that the planners and financiers of this brutality are deeply embedded in our communities , '' President Kenyatta said during an address to the nation in the aftermath of the massacre . Meanwhile , Sheikh Khalif insists his Madrassa has nothing to do with Mohamud 's extreme , violent ideas . `` This man is a dangerous man , a killer , a criminal , '' he says . But he was also once a neighbor . And so Kenyans must now look within to tackle this very real threat to the country 's -- and the region 's -- stability .
The attack at a Garissa mosque last night killed 147 people , mostly civilians . Mohamed Mohamud taught at a mosque in the Kenyan countryside . villagers fear the rise of homegrown violence in the African country .