Nairobi, Kenya (CNN) -- At least 28 people were injured Monday, four of them critically, in an explosion in downtown Nairobi, according to Kenyatta National Hospital.
Prime Minister Raila Odinga said he believes the blast was "act of terror," but hospital and police on the scene said they suspect an electrical fire was the cause.
The hospital said injuries were "not consistent with a grenade or bomb blast."
But Dennis Onyango, an aide to Odinga, said the level of damage was too much for a fire.
The electrical company said it was not possible that there had been an electrical fault.
The prime minister said it was an attack "perpetrated by the same forces Kenyan security is fighting abroad."
Kenya has troops in neighboring Somalia fighting Al-Shabaab, an Islamist militia that controls parts of the country.
The blast occurred around 1:15 p.m. at a shopping center. Witnesses said the earth shook and pieces of metal rained down on the street.
Neighboring buildings, including one housing a broadcaster, were rattled. The network showed people running through streets afterward.
"I looked up and saw a huge piece of metal coming down," said Jessie Waweru, who was doing business in the area. "It was red hot!"
"I just heard a loud blast," said student John Mwangi. "It went 'bang'! I just started running with other people. There was confusion everywhere."
"I was scared and dropped down wondering what happened," said Dennis Mutua, a driver. "Then I saw smoke."
The small shopping center has a variety of stores, mostly selling clothes and shoes.
Charred remnants of clothing could be seen throughout the area.
Windows were blown out in buildings across the street.