THUNDER BAY (From news reports) -- Ontario's labour ministry has issued numerous instructions to Thunder Bay Pulp and Paper and one of its contractors following a fiery explosion Friday that injured four people, including two who were flown to a Toronto hospital burn unit for treatment.

Video of the incident shows that a fireball - lasting two or three seconds - erupted from a building where the hog fuel feed system is located.

Hog fuel is waste wood fibre that's burned to produce electricity.

The Ministry of Labour, Immigration, Training and Skills Development revealed Tuesday that it has issued 11 "requirements" to the mill owner, and 10 requirements to Skyway Canada, a contractor that offers a variety of industrial services.

But the ministry said no further details will be released while the investigation is still underway.

A Thunder Bay Fire Rescue spokesperson has said he was told a bearing overheated prior to the explosion.

"It sounds like the machine started to shake, and then all the dust went in the air, and it was just the right mixture to ignite it," acting platoon chief Robin Roy said on Saturday.

Kent Ramsay, president of Thunder Bay Pulp & Paper, issued a statement Tuesday, saying "Our thoughts remain focused on the four contractors who received treatment for injuries sustained in the incident. We are working with their employer to ensure they are supported in whatever ways are required to ensure their speedy and full recovery."

Ramsay said TBPP operates within a culture of safety, and that it will continue to assess and implement the results of the investigation into the incident in conjunction with the labour ministry and the Office of the Fire Marshal.