The attack comes amid heightened tensions across Kenya. Last Saturday, suspected al Shabaab militants from Somalia killed at least 28 people in northern Kenya's Mandera County after ambushing a passenger bus.
Kenyan has been targeting al Shabaab positions in Somalia as the country pursues the militants who have carried out the bus attack. Kenya first sent troops into Somalia in October 2011.
Police Commander Musa Yego told Reuters by phone that local intelligence officials believed al Shabaab militants had staged the attack in Garissa.
He said the police force was on "high alert."
Al Shabaab claimed responsibility last year for an attack on Nairobi's Westgate shopping mall in which at least 67 people were killed, and for attacks in the Lamu region in June and July in which at least 65 people died.
The group, whose leader Ahmed Abdi Godane was killed in September, has vowed to drive Kenyan and other African Union peacekeeping troops out of Somalia.