Kenyan police are investigating an explosion at a shopping complex in Nairobi's business district which left at least 28 people wounded.
Police spokesman Eric Kiraithe said on Monday that anti-terrorism police were combing the blast site in the city centre for clues, appearing to row back on an earlier suggestion by the police commissioner that a massive electrical fault might be to blame.
"The investigating team is exploring the possibility that the blast was caused by criminals using an improvised explosive device," Kiraithe said in a statement.
Shopkeepers had earlier told the Reuters news agency independently that they saw a man drop a bag inside the trading centre moments before the blast.
"He came into the shop twice, looking at t-shirts. He said he didn't have money so he left. Then he came back," said Irene Wachira.
"[He was] three shops away from where I was. He left a bag and a few moments later we had an explosion. The roof caved in and debris started falling on us," Wachira said.
Al Jazeera's Peter Greste, reporting from Nairobi, said that most of the devastation seemed to be "in a small and confined space" and that security sources had told him that evidence suggested a grenade might have been used.
"So although we don't have an official explanation for the explosion, the indications are that it might have been some kind of bomb attack," said our correspondent.
Raila Odinga, the Kenyan prime minister, also suggested the blast had been a deliberate attack, calling it an "act of terrorism".
"This is a heinous act, we are under threat but we will not be cowed," he said.
The Kenya Red Cross said on Twitter that 28 people had been admitted to hospital with four patients in a critical condition.
A spokesman for Kenya Power, the country's sole electricity distributor, said initial investigations had ruled out any electrical malfunction.
Kenya has experienced a series of attacks in recent months, both in Nairobi and the port city of Mombasa, blamed on Somalia's al-Shabab and their sympathisers since October when Kenya sent troops into Somalia to fight the Islamic armed group.