By Cyrus Ombati
Police and witnesses say the Wednesday evening blast occurred as a group of worshippers were leaving a nearby mosque after their prayers.
Police say it is not clear who hurled the grenade that exploded few metres away from the WFP main office.
Outgoing North Eastern PPO Philip Tuimur say no arrests have been made. Tuimur said bomb experts are at the scene accessing the situation.
The blast occurred two days after one person was killed in a grenade attack on police vehicle in Garissa.