Ammonium nitrate piled high in a flammable wooden bin caused the double explosion that rocked West with the force equivalent to about 20,000 pounds of TNT, investigators said Thursday, but the cause of the fire that ignited the bins remains 
undetermined.

Federal and state investigators said they could not rule out some possible causes of the April 17 West Fertilizer Co. fire — arson, electrical wiring problems or a battery-powered golf cart stored near the ammonium nitrate fertilizer.

They did rule out speculation that it was caused by weather, spontaneous ignition, smoking or the rekindling of a fire earlier in the day.

At a news conference Thursday afternoon, officials with the federal Bureau of Alcohol, Tobacco, Firearms and Explosives also said they would not discuss any speculation of a link between the disaster and the May 9 arrest of former West emergency volunteer Bryce Reed on charges of possessing bomb-making materials.

The McLennan County Sheriff’s Office and Texas Rangers continue an investigation of any possible criminal activity in the blast but say there is no evidence linking Reed to the explosion.

The news conference marked the end of a monthlong scene investigation that consumed 20,000 personnel hours and cost more than $1 million.

Scores of investigators from
28 agencies collected and studied debris from as far as 2.5 miles from the scene, conducted hundreds of interviews and hand-sifted 300,000 pounds of corn and sorghum, kernel by kernel. They built models of the buildings and electrical systems.

ATF and state fire officials will continue analyzing the evidence in coming months.

“We will leave no stone unturned to make sure that we do everything we can do to determine what caused this fire and explosion and will continue to investigate,” State Fire Marshal Chris Connealy said.

Officials previously had concluded that it was ammonium nitrate that detonated in the blast that killed 15 people, including 12 first responders. It injured 200 people and damaged more than 300 homes.

Details on explosion

New details about that explosion emerged Thursday.

Investigators think the wooden fertilizer bins caught fire, and the heat caused the fertilizer to create flammable gases. The gases accumulated under pressure in the tall column of the bin, and falling equipment and debris created the shock necessary to set off a portion of the fertilizer that had become “highly sensitized,” Assistant State Fire Marshal Kelly Kistner said.

The first blast set off another portion of the ammonium nitrate a few milliseconds later, a sequence confirmed by seismic readings miles away at Lake Whitney.

Kistner said 28 to 34 tons of ammonium nitrate exploded, leaving 20 to 30 tons that did not explode. Another 100 tons were sitting in a boxcar nearby that did not explode.

The total amount of ammonium nitrate fertilizer on hand was significantly less than the 270 tons the company reported in late 2012, according to
Kistner.

The bin that exploded was about 20 to 24 feet tall, 10 feet wide and 30 feet deep, said Daniel Horowitz of the U.S. Chemical Safety Board.

He said national firefighting organizations recommend fire codes that require explosive materials to be stored in nonflammable containers, but he said it appears neither the state nor the county have adopted those codes.

The safety board is doing its own investigation of the incident to determine what safety measures are needed to prevent incidents like this elsewhere.

“This is the worst amount of damage to a community the Chemical Safety Board has ever seen,” Horowitz said. “We simply can’t have explosions like this happen again.”

John McCoy, an attorney for West Fertilizer Co., said the company is doing its own investigation and fully cooperating with investigators.

“Out of respect to the various agencies and to assure that their investigations are not compromised, we will not offer any additional comments or opinions at this time,” McCoy said in an emailed statement late Thursday. “Our thoughts and prayers continue to be with the families and friends of those lost in this tragedy.”

Steve Vanek, West’s mayor pro tem, said he was pleased with the work of investigators, describing their presentation as “very thorough.”

He said he had known that the cause may not be determined due to the sheer devastation from the blast, but hopes investigators one day can find a definitive answer.

Vanek noted that it could be “weeks, or months, or years,” if ever, before a cause is identified.

Vanek said he doesn’t personally think the fire was intentionally set, saying he thinks it was “purely, 100 percent an accident.”

Mayor Tommy Muska said he doesn’t think Reed was involved in the incident but was simply a “nuisance” and a “distraction.”

Earlier meeting

Before officials announced the results of the investigation to the media, about 150 people attended a meeting at West First Baptist Church to hear ATF officials give a preview of their findings. The meeting included family members of those lost in the blast, representatives from the ATF and State Fire Marshals’s office, volunteer firefighters and 
others.

Billy and Beverly Uptmor, parents of Buck Uptmor, a firefighter who was killed in the explosion, said the undetermined cause was frustrating news.

“I’d like to know, but what happened happened and you can’t change that,” said Billy Uptmor, 68, a retired machinist.

Beverly Uptmor said her family still is in shock and hasn’t come to terms with the fact that Buck is gone.

“He was just a large presence in our family,” she said. “He helped Billy with the cattle at the farm. It just seems like he should be calling or driving up in the driveway. It just doesn’t seem real.”

The Rev. John Crowder, pastor of West First Baptist Church, who lost his home in the explosion, said he thinks the families of those killed appreciated officials for keeping them in the loop and holding the meeting.

He said the community’s first priority remains trying to attend to the emotional needs of those who suffered the most, those who lost loved ones.

Staff writer Tommy Witherspoon contributed to this story.