The explosion blew the roof off an office building known as the Assanands Building Monday.
Sources at the Kenya Police Criminal Investigation Department told Kenya's Capital FM News Tuesday initial analysis of the debris indicated the presence of ammonium nitrate, a fertilizer ingredient that can be used in making explosives.
"There are traces of fertilizer at the site of the blast. There is also the strong smell of fuel," a senior CID official told the broadcaster.
Prime Minister Odinga called the blast a "terrorist" attack, and police said it most likely was caused by a homemade bomb.
Capital FM News said 23 of the 38 people injured in the explosion had been released from Kenyatta National Hospital in Nairobi, and that two remained in critical condition.
The incident has been linked to a series of attacks in Kenya since Kenyan troops were sent into Somalia to fight al-Shabaab militants. Most of the attacks have been tied to the terrorist group, the newspaper Business Daily reported Tuesday.