There were signs that a natural gas well was not fully sealed before it exploded in January, killing five workers, the U.S. Chemical Safety Board (CSB) said in an updated report on the accident Aug. 16.

The incident, which took place at a Pittsburg County, Oklahoma, well being drilled for producer Red Mountain Energy by drilling contractor Patterson-UTI, was the deadliest drilling accident since the 2010 Deepwater Horizon rig explosion, which killed 11 people.

The CSB’s chronology of events in the hours leading to the fatal blowout and fire showed a drill pipe was removed before it was fully emptied of the mud used to seal a well. Later, a mud pit at the site accumulated 107 barrels of the material, providing “an indication of a possible gas influx in the well,” the report said.

The report does not specifically say what caused the blast. The CSB investigates major industrial accidents and makes recommendations to prevent future incidents.

Patterson-UTI Drilling said in a statement that it is cooperating with the CSB. “We value the perspective of outside stakeholders and will be working with the CSB to understand their potential recommendations when they are available at the conclusion of the investigation,” a spokeswoman said.

An attorney representing the family of one victim of the explosion said the report illustrates the safety failures that led up to the explosion.

“Ten hours before it blew, there were warning signs,” said Michael Lyons, an attorney representing the family of Parker Waldridge, a 60-year-old well-site consultant killed in the accident. "At this point, the well should have been shut in.

"The [mud] pit gains were a big deal,” Lyons said.

Onshore