Law enforcement experts said a suspect's intent could be important as prosecutors decide whether to pursue arson, murder or domestic terrorism charges.
Elder declined Wednesday to discuss specifics about suspects, possible charges or motive for the April 17, 2013, incident, one of Texas' deadliest industrial disasters.
Elder said the investigation had in recent months eliminated two other potential causes that remained — faulty electronic wiring at the building and a short-circuiting golf cart.
The probe cost $2 million and included more than 400 interviews, a fire-scene examination and "extensive scientific testing" at an ATF fire research laboratory.
Matt Orwig, the former U.S. attorney for the Eastern District of Texas, said whatever the motive and charges, anyone convicted probably faces at least life imprisonment.
"What this person has done is commit mass murder by turning that building and its contents into a weapon of mass destruction," Orwig said.
Investigating a complex explosion scene like West requires time and extensive scientific lab work, said Mike Bouchard, a retired assistant director for ATF field operations who supervised the investigation of the Sept. 11 attack on the Pentagon.
Orwig, the former U.S. attorney, said the decision to go public signals the highest levels of the Justice Department believe there is "strong evidence to support what they're saying occurred."
