A blast that ripped through a building in downtown Nairobi Monday, injuring 33 people, was caused by an improvised explosive device, Kenyan authorities said Tuesday.

One man has been arrested so far in connection with the explosion and is currently being interrogated by police.

Members of the public saw him running from the scene and he could have been either looting, doing some kind of other criminal activity or in shock, police spokesman Eric Kiraithe told the Bloomberg news agency.

Kenyan police were unable to confirm whether or not the IED detonation was connected to the terrorist organization Al-Shabaab, which threatened to carry out attacks in the country after Kenyan troops were deployed to southern Somalia last October to fight the group, which is based there.

Following Al-Shabaab's proclamation, Nairobi was hit by a series of grenade attacks, killing at least 40 people. The Kenyan government blamed Al-Shabaab for the attacks.

Al-Shabaab (Youth in Arabic) is a militant Islamic group that controls southern and central regions of Somalia, and has suspected ties to the higher-profile terrorist organization Al-Qaeda.