The July 27 blast at the Chempark chemical complex in the western city of Leverkusen, which also injured 31 workers, was heard several kilometres away and rattled the windows of nearby homes.

It sent up a large cloud of black smoke that prompted authorities to urge locals to stay indoors, although later examinations showed no danger to residents’ health.

The interim findings, announced by the Cologne district government, suggest that a chemical reaction “probably” caused waste liquid in storage tank 3 to self-heat and rapidly become warmer, leading to a build-up of pressure it was unable to withstand.

“The whole process happened so quickly that the safety mechanisms were no longer able to dissipate the pressure. When the pressure exceeded the design pressure of the container, it exploded,” the interim report is quoted as saying.

The explosion then triggered a large blaze at the park’s waste storage and incineration site that took firefighters hours to put out.

Chempark operator Currenta said in a statement that the early findings match its own investigations.

The Cologne district government said it was awaiting further expert analysis before final conclusions could be made about the cause of the blast.

Prosecutors in Cologne have opened an investigation against unknown persons on suspicion of involuntary manslaughter and causing an explosion through negligence.

All those killed and injured in the explosion worked at the chemical site.

The blast area, in Leverkusen’s Buerrig district, was separate from Chempark’s main industrial park that houses numerous companies including Bayer, Lanxess and Evonik Industries.

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