Police announced that an improvised explosive device caused a blast that ripped through a building full of small shops in downtown Nairobi, with one official saying it may have been a fertilizer bomb.
The official, who spoke Tuesday on condition of anonymity, told The Associated Press that the smell of ammonia at the scene of Monday's explosion on Moi Avenue indicated the possible presence of a fertilizer bomb, which is commonly made of ammonium nitrate and fuel oil.
The FBI joined the investigation into the attack that wounded 33 people, including a woman who blamed the blast on a "bearded man" who left behind a bag shortly before the detonation.
Kenya's police spokesman, Eric Kiraithe, said police concluded on Tuesday that the blast was caused by an IED. Kiraithe said officials couldn't yet definitely blame the Somali militant group al-Shabab, an Islamist group that has links to al-Qaida.
Al-Shabab threatened in October to bring down Nairobi skyscrapers and bragged about its July 2010 bomb attacks in Kampala, Uganda, that killed 76 people. Al-Shabab issued the threat against Kenya after Kenyan troops moved into Somalia to attack al-Shabab fighters.
FBI agents helped analyze materials at the blast site at Kenya's request, said U.S. Embassy spokesman John Haynes. A group of FBI agents could be seen at the site Tuesday, sifting through debris and packing samples with members of Kenya's Anti-Terror Police Unit.
The intelligence firm IntelCenter said al-Shabab militants bragged about acting as journalists and conducting interviews of survivors after the blast, posing 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 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.
There were fears that the attack could be a dry run for a major terrorist bombing.
Tuesday's explosion follows several grenade attacks the Kenyan government has blamed on al-Shabab. At least 40 civilians have been killed in the grenade attacks since October, which police attribute to Kenyan sympathizers of al-Shabab.
At the site of the Moi Avenue explosion, some Kenyans expressed anger Tuesday over what they said was the police's inability to protect them. Marketing consultant Lucas Okwany, 46, said many Kenyan police officers are more interested in collecting bribes than carrying out police duties.
"I am scared because in marketing you have to go into all these buildings to look for work, and any of them could now be a target," he said.
Kenya's police force is constrained by poor resources and low pay, which encourages the bribe-seeking. Few police in Kenya have cars, and those who do are given little fuel.
The rising insecurity in Kenya has also impacted the hotel business. Hotel workers, who could not give their names for fear of losing their jobs, say many hotel rooms are empty because business travelers have been scared away by the attacks.
Mike Macharia, the chief executive of the Kenya Association of Hotel Keepers and Caterers, said there has been a drop in bookings but that hotels can still recover with the help of aggressive marketing and increased security.
Al-Shabab has not yet made any public comment on the attack.