Kenyan Prime Minister Raila Odinga says a blast in central Nairobi on Monday was an act of terrorism, contradicting police who blamed the explosion on an electrical fault.

Speaking at the blast site a few hours the explosion occurred, Mr. Odinga said Kenya is “under threat” but will “not be cowed.”

The explosion Monday injured at least 27 people and caused significant damage to a building in a Nairobi business district.

Police Commissioner Matthew Iteere told reporters the blast was probably caused by an electrical malfunction.

However, the assistant minister for internal security, Orwa Ojode, said police are investigating the possibility of a grenade attack.

Kenya has been hit by a string of grenade attacks that authorities blame on the Somali militant group al-Shabab and its sympathizers.

Kenya sent troops into Somalia last October to fight al-Shabab, which it blamed for a series of cross-border kidnappings.