The explosions at the West Fertilizer plant in West caused damages in the community and were felt up to 50 miles away, authorities said.
On Thursday, State Fire Marshal Chris Connealy said investigators have not been able to rule out three possible causes: a spark from a golf cart, an electrical short or an intentionally set fire, CNN reported.
"The cause cannot be proven to an acceptable level," Connealy said.
Connealy promised to "leave no stone unturned to make sure everything is done" in the investigation.
About 30 local, state and federal agencies have been working on the investigation, NBC News reported.
West Fertilizer said in a statement it had no comment other than "the authorities repeatedly emphasized that their investigation continues, as does ours."