In 2016 the company, Wood Treatment, was carrying out instructions from the Health and Safety Executive (HSE) to fit new explosion relief panels to an asbestos cement-clad warehouse at the site in Bosley, Macclesfield, after the fatal blast.
During a follow-up inspection to assess the work's progress, two HSE inspectors found the company's employees had removed two of the warehouse's asbestos cement wall panels and had used power tools to cut them to size before replacing them, breaching the Control of Asbestos Regulations.
The HSE said that though a contractor had been appointed to install the explosion relief panels, its workers were not trained to do so, and Wood Treatment's employees carried out the work themselves. They also used parts of old asbestos sheets to repair holes in one of the walls.
Manchester and Salford Magistrates' Court was told that Wood Treatment was "fully aware" of the presence of asbestos. It had an asbestos survey and action plan, though these were not shared with employees and measures were not implemented to manage the risk of exposure.
Wood Treatment pleaded guilty to breaching s 2(1) of the Health and Safety at Work Act. It was fined £2,000 plus £3,000 costs.
HSE inspector Ian Betley said: "Asbestos should only be removed by specialist contractors. Wood Treatment put workers at risk by not following the correct safety procedures."
Four workers died in the blast on 17 July 2015. In December that year the HSE served several prohibition notices on Wood Treatment, relating to its failure to maintain the electrical system and its failure to eliminate wood dust in the remaining buildings.
On the third anniversary of the accident, detective inspector Paul Hughes from Cheshire Police major investigations team said in a statement: "We are continuing to provide support to the families of Derek Moore, 62, Dorothy Bailey, 62, William Barks, 51, and Jason Shingler, 38, who sadly lost their lives and they are being kept fully updated on the progress of the investigation.
"The joint investigation is being managed by Cheshire Police and the Health and Safety Executive. Due to the highly complex and very detailed nature of this case it will take time to come to a conclusion but we are fully committed to doing everything we can to carry out as thorough an investigation as possible into what happened that day.
"Detectives have been liaising with the Crown Prosecution Service for advice and we will provide a further update once we have submitted our file to them."