US fertiliser plant blast

Firefighters conduct search and rescue of an apartment destroyed by an explosion at a fertilizer plant in West, Texas, Thursday, April 18, 2013. A massive explosion at the West Fertilizer Co. killed as many as 15 people and injured more than 160, officials said overnight. (AP Photo/LM Otero)

Firefighters conduct a search and rescue of an apartment destroyed by an explosion at a fertilizer plant in West, Texas, Thursday, April 18, 2013. A massive explosion at the West Fertilizer Co. killed as many as 15 people and injured more than 160, officials said overnight. (AP Photo/LM Otero)

A destroyed car sits as firefighters conduct a search and rescue of an apartment complex destroyed by an explosion at a fertilizer plant in West, Texas, Thursday, April 18, 2013. A massive explosion at the West Fertilizer Co. killed as many as 15 people and injured more than 160, officials said overnight. (AP Photo/LM Oter

Firefighters us flashlights to search a destroyed apartment complex near a fertilizer plant that exploded earlier in West, Texas, in this photo made early Thursday, April 18, 2013. (AP Photo/LM Otero)

The remains of a home burn early Thursday morning, April 18, 2013, after a fertilizer plant exploded Wednesday night in West, Texas. The massive explosion killed as many as 15 people and injured more than 160, shaking the ground with the strength of a small earthquake and leveling homes and businesses for blocks in every direction. (AP Photo/LM Otero)

Firefighters use flashlights early Thursday morning, April 18, 2013 to search a destroyed apartment complex near a fertilizer plant that exploded Wednesday night in West, Texas. The massive explosion killed as many as 15 people and injured more than 160, shaking the ground with the strength of a small earthquake and leveling homes and businesses for blocks in every direction. (AP Photo/LM Otero)