The fertilizer facility stores and distributes anhydrous ammonia, a fertilizer that can be injected into soil. It also mixes other fertilizers.
Plant owner Donald Adair released a statement saying he would never forget the "selfless sacrifice of first-responders who died trying to protect all of us."
One of the plant employees was also killed responding to the fire, Adair said.
Federal investigators and the state fire marshal's office planned to begin inspecting the blast site Friday to collect evidence that may point to a cause.
Franceska Perot, a spokeswoman for the Bureau of Alcohol, Tobacco, Firearms and Explosives, said investigators would begin at the perimeter of the explosion and work inward toward the destroyed fertilizer company.
Cornyn and Sen. Ted Cruz, who toured the town Friday, said they would wait for more information about the explosion before considering whether there should be more regulation of anhydrous ammonia.
The accident forever changed the community's landscape. An apartment complex was badly shattered, a school set ablaze and a nursing home left in ruins. At West Intermediate School, which was close to the blast site, all of the building's windows were blown out, as well as the cafeteria.
Marek was teaching a high school youth group when the blast shook the room. The lights went out, and a student's phone lit up with a text message that there was an explosion at the fertilizer plant. He told Marek his brother's truck had been picked up and hurled into his family's house.
Marek spent the next couple of hours wondering if she knew anyone who might be at the plant. Then Uptmor's wife called.
"She said, 'Have you heard from Buck? She told me they had called him up there, and she couldn't get a hold of him," Marek said.
They spent the next few hours frantically searching for the father of three, who coached baseball, played drums in a band and whose phone was always ringing with people seeking help. Sometimes it was a truck stuck in a ditch or a house that flooded or a neighbor who needed a hand moving furniture.
Every time, Marek said, Uptmor would go.
"Why did they have to call him? He was safe at home with his family," Marek said. "But you know, if he hadn't gone, he wouldn't have been Buck."
Associated Press writers Will Weissert and Christopher Sherman in West, Juan Carlos Llorca in Dallas and video journalists John L. Mone and Raquel Maria Dillon in West contributed to this report.