Investigators announced Wednesday that the fire that ultimately led to the explosion of the West Fertilizer plant was "incendiary in nature" and a criminal act.

At a noon press conference in West, Special Agent in Charge Rob Elder of the Bureau of Alcohol, Tobacco, Firearms and Explosives said the fire that started in a seed building had been ruled incendiary after an investigation "ruled out all reasonable, accidental, and natural causes".

Elder refused to comment on any specific evidence that led to this conclusion, but said the investigation was one of the most expensive and exhaustive in the agency's history. He said interviews with over 400 witnesses and extensive scientific testing ultimately ruled out every hypothesis except an incendiary scenario.

When asked how close the agency is to determining a suspect, Elder said they are "moving in the right direction", but would not give details.

Elder was asked if former West paramedic Bryce Reed - convicted of attempted obstruction of justice and conspiracy to manufacture a destructive device in December of 2013 - was a suspect. Elder said Reed "has served his time" and was not currently a suspect.

Reed was arrested three weeks after the West explosion after investigators found explosive materials in a home in Abbot that Reed reportedly hid there after a eulogy for a firefighter killed in the blast mentioned that the deceased and Reed enjoyed creating and detonating explosives.

Elder said the ATF is offering a reward of up to $50,000 for information leading to the arrest of the person or people responsible, and that Waco Crime Stoppers is offering an additional $2,000. Information may be submitted via phone at (254) 753-4357 or online at wacocrimestoppers.org.

The April 17, 2013 explosion at West Fertilizer killed 15 people, including 12 emergency responders. It left a crater 12 feet deep and 93 feet wide and sent debris flying as far away as 2.5 miles.