Kenyan police say an explosion that wounded at least 38 people in Nairobi Monday was caused by a bomb.

Police had initially said the explosion, in a building full of small shops, resulted from an electrical malfunction. However, police spokesman Eric Kiraithe said Tuesday the blast was caused by an improvised explosive device.

Some investigators said evidence at the scene suggested the device may have been a fertilizer bomb.

There has been no claim of responsibility but the attack has raised concerns about al-Shabab, a Somali Islamist group known to have sympathizers in Kenya.

Kenyan authorities have blamed al-Shabab supporters for a series of grenade attacks in the country. Kenya sent troops into Somalia last October to fight al-Shabab, after a series of cross-border kidnappings.

On Monday, Kenyan Prime Minister Raila Odinga denounced the trade center explosion as an act of terrorism. He said Kenya is under threat but will “not be cowed.”