Kenya’s police say an explosion which injured 33 in the capital, Nairobi, may have been caused by a bomb, after initially blaming an electrical fault.The blast tore apart a shopping complex on Moi Avenue at lunchtime.A witness has said a bag was abandoned next to her just before the explosion.Prime Minister Raila Odinga has said the blast had a terrorist link. Militant group al-Shabab has repeatedly threatened to stage revenge attacks after Kenya sent troops to Somalia.There has been a string of grenade and bomb blasts across Kenya in recent months, killing several people.Shoes and clothes scatteredFive of those injured are said to be in a critical condition.Police spokesman Eric Kiraithe told the BBC he no longer believed an electrical fault had caused the blast, as police commissioner Mathew Iteere had initially suggested.In a statement, Mr Kiraithe said the cause of the explosion had not been established."The investigating team is exploring the possibility that the blast was caused by criminals using an improvised explosive device,” he said."Initial examination of the scene indicates that the possibility of a conventional bomb is remote.”Mr Odinga told reporters at the site of the blast: "This is terrorism... this is a heinous act, we are under threat, but we will not be cowed.”Kenya Power, the country’s sole electricity distributor, has also ruled out any electrical malfunction as the cause of the blast.The BBC’s Kevin Mwachiro says glass and shoes and clothes from the small shops inside the building were scattered across the street.