The other five companies were Cincinnati-based The Pendleton Construction Group, Messer Construction Co., D.A.G. Construction Co., Jostin Construction Inc., Triversity Construction Company.
Messer officials told the Cincinnati Enquirer it is working with OSHA but will question the findings.
Thirteen workers were hurt when the Cincinnati casino floor collapsed while under construction. Work was initially suspended at the site while the investigation took place. At the time, officials said a beam buckled as workers poured concrete at the site.
Some of the injuries were serious, but the men who were injured have recovered.
Work at the site has since resumed.