Nairobi - A fertilizer bomb could have caused the blast that ripped through a building full of small shops, an official told The Associated Press on Tuesday as the FBI joined the investigation.
The official, who spoke on condition of anonymity, said the smell of ammonia at the scene of Monday's explosion on Moi Avenue indicates the possible presence of a fertilizer bomb, which is commonly made of ammonium nitrate and fuel oil.
Among the 33 people wounded was a woman who blamed the blast on a "bearded man" who left behind a bag shortly before the detonation.
The intelligence firm IntelCentre said militants from the al-Qaeda-linked Somali group al-Shabaab bragged about acting as journalists and conducting interviews of survivors after the blast. IntelCentre said the development poses significant challenges to security forces and legitimate members of the media covering attacks.
The explosion sent dark smoke billowing out of a one-story building on the downtown avenue named after Kenya's second president. The blast peeled back the front corner of the building's aluminum roof, shattered windows in the building and scattered shoes, clothes and other wares on the ground. A high-rise building with a glass exterior next door was largely untouched.
Al-Shabaab threatened in October to bring down Nairobi skyscrapers and referenced the July 2010 bomb attacks they masterminded in Kampala, Uganda, that killed 76 people. Al-Shabaab issued the threat against Kenya after Kenyan troops moved into Somalia to attack al-Shabaab fighters.
One shop worker wounded in the blast, Irene Wachira, said from her hospital bed on Monday that a bearded man came to a nearby stall three times and acted as if he were interested in buying something. Wachira said the third time he came with a bag that he left behind. The blast occurred shortly afterward, she said.
Wachira described the man as "Arabic-looking" because of his relatively light skin. A doctor told AP that another person wounded in the blast said a Somali-looking man left behind the bag. The doctor said he could not be quoted by name.
Al-Shabaab has not yet made any public comment on the attack.
Police officials first indicated that Monday's explosion could have been caused by some sort of electrical malfunction but the prime minister said it was deliberate.
"This is a heinous act," Prime Minister Raila Odinga said while visiting the scene of the blast. "They want to scare us. But we will not be scared."