One month after a fire triggered a massive blast at a Texas fertiliser plant that killed 15 people, officials had to declare the cause of the blaze as "undetermined".
Investigators narrowed the number of possible causes to three: a problem with one of the plant's electrical systems, a battery-powered golf cart, and a criminal act.
They ruled out a wide number of others, from a rail car on site loaded with fertiliser to someone smoking.
But they could not say with certainty what caused a fire on April 17 inside the seed and fertiliser building at West Fertiliser, in West, a tiny Texas town near Waco, previously known for its Czech bakeries and heritage.
Kelly Kistner, the Texas assistant state fire marshal, said the fire caused stored ammonium nitrate to change states, while also causing debris in the wooden building to begin to fall.
The blast was actually two explosions, a small one that occurred about 20 minutes after the fire was reported, followed by a larger one a split second later.
About 28 to 34 tons of ammonium nitrate stored in the plant exploded. Another 20 to 30 tons stored on site, along with a rail car carrying 100 tons of ammonium nitrate, did not explode, officials said.
The power of the blast was equivalent to about 20,000 pounds of TNT.
Among the dead were 10 members of the emergency services and two people who had joined in to fight the fire. The blast registered as a small earthquake and left a crater 93 feet wide and 10 feet deep. It destroyed an apartment building, homes and parts of schools nearby.
The golf cart was parked in the room where investigators believe the blast began, officials said. The golf cart's batteries hold a charge and when they fail, they can ignite the materials around them, officials said. They found two pieces of the cart, a brake pad and the axle.