LINCOLN, Neb. (DTN) -- The explosion that killed three people at the Horizon Biofuels plant in Fremont, Nebraska, in July was most likely ignited by wood dust, the U.S. Chemical Safety and Hazard Investigation Board said on Wednesday in a report updating the ongoing investigation.

The investigation is being hindered by a facility that remains unsafe to access, according to the agency.

The company produced wood pellets and animal bedding from scrap wood and the facility had a dust collection system to help control dust.

"At the time of writing this update, the Horizon Biofuels facility is still not safe for access," the CSB said.

"Combustible materials at the facility have continued to smolder weeks after the incident and the city of Fremont has advised people to maintain a safe distance from the facility due to the potential for the structurally compromised building to collapse, which has prevented the CSB from approaching the building so far."

Steve Owens, chairman of the CSB, said in a statement on Wednesday the incident could have been prevented.

"This terrible tragedy should not have happened," he said.

The explosion killed Horizon employee Dylan Danielson and his 8-year-old and 12-year-old daughters. The CSB said this week that Danielson initially survived the explosion but was trapped inside the building and died.

"Preliminary evidence points to a combustible wood dust explosion, a well-known -- and completely avoidable -- hazard in wood processing."

In February 2025, the Nebraska Department of Environment and Energy conducted an air quality investigation at the plant in response to a complaint regarding wood dust blowing into adjacent properties, the CSB said on Wednesday.

A Horizon Biofuels representative told the CSB that, in response to the complaint, the company temporarily shut down the plant, the agency said, and resolved an issue with one of the cyclones which is a device that controls wood dust generated from the process, before restarting operations as normal.

The CSB said dust can accumulate on surfaces and lie undisturbed for years. A fire or explosion can shake dust loose and cause it to disperse throughout a building, the agency said. When dust reaches a certain concentration, it can ignite.

"The CSB previously investigated eight combustible dust explosion incidents that spanned multiple industries and commodities," the agency said in its investigation update.

"In 2006, the CSB also published a combustible dust hazard study that analyzed 281 combustible dust incidents between 1980 and 2005 that killed 119 workers, injured 718 and extensively damaged industrial facilities."

According to the report, the general manager of the facility was working from home on the day of the incident.

"Upon learning of the explosion, at 12:06 p.m., the general manager called the day shift operator on his cell phone, who reported that he was trapped," the report said.

"Soon after, the day shift operator called his wife and told her he was pinned down. Witnesses working in neighboring businesses reported that they approached the building and heard the operator call for help. Some of the witnesses entered the collapsed warehouse area and attempted to rescue him without success."

In addition, the CSB said employees said the facility had "operated normally" the day before the incident and no issues were reported during the night shift before the explosion.

Todd Neeley can be reached at todd.neeley@dtn.com

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