North Eastern deputy head of the Criminal Investigations Department (CID) Ngatia Iregi said that the suspect had confessed of involvement in the killings.
Soldiers who survived the attack had also identified the Kenyan man of Somali origin after he was arrested at Hulugho region at the border with Somalia.
The killing led to an immediate massive security operation in Garissa, sparking protests from locals who claimed they were being unfairly targeted by the police and military.
MPs from the region including Farah Maalim (Lagdera) and Aden Duale (Dujis) accused the military of participating in human rights violations and causing damage to commercial property in the town.
Prime Minister Raila Odinga later toured the largely restive town and condemned the attacks saying the government will ruthlessly deal with the perpetrators of the violence against innocent people.
The PM remained categorical that the government will not let terrorism take root in the country adding that all terror groups will meet the full wrath of the law.
The Cabinet has also ordered the establishment of an inquiry into the killings and the subsequent destruction of property in Garissa.