Prime Minister Raila Odinga has said the government will use every available resource to fight terrorists within its borders.
PM Odinga reiterated this at the scene where 30 people were injured, four in critical condition, in the 1:10pm explosion that rocked Assanands House next to Mt Kenya University Campus along Nairobi's Moi Avenue street.
"We will make sure all residents in Kenya aresafe," he said.
Earlier, Police Commissioner Mathew Iteere stated that the blast's probable cause an electrical fault within the building.
However, Kenya Power said the blast could not have occurred as a result of an electrical fault since all gadgets at the scene were functional.
"The affected building has no ground mounted transformer inside it or outside that would explode," Kenya Power said in a statement.
The cause of the explosion was further put in doubt as firemen at the scene came across a two feet crater at the floor of the the building.
In an interview with NTV, Mr Iteere said the police have not found any evidence relating to a grenade attack.
Later, in a statement to the newsroom, Police Spokesman Eric Kiraithe said a team of Anti-Terrorism Unit and the Criminal Investigations Department were investigating all possibilities relating to the blast.
"The investigating team is exploring the possibility that the blast was caused by criminals using an improvised explosive device," he said.
Internal Security assistant minister Orwa Ojode who arrived at the scene later said "police are investigating the possibilities of a grenade attack."
According to an eye witness, there was a huge blast as debris flew in different directions injuring people in the environs.
The powerful explosion tore the building roof and shattered windows across the street. Occupants of surrounding buildings were evacuated.
The explosion has disrupted activities along the street as Military Police cordoned off the area and forcefully evacuated curious onlookers.
So far, Kenyatta National Hospital (KNH) has reported receiving 27 casualties.