CALVERT CITY, Ky. (AP) — Federal investigators say a fatal towboat explosion in Kentucky earlier this year was caused by people welding and cutting near flammable gases.

The Paducah Sun reports the Occupational Safety and Health Administration released its investigation into the Calvert City explosion Tuesday. A towboat being repaired at First Marine Dry Dock & Boat Repair in January exploded, killing three people and injuring six more.

OSHA cited First Marine Dry Dock & Boat Repair, contractors Joe Rupcke and Thermal Control and Fabrication and staffing companies HUTCO and Wise Staffing Group with 55 total violations. Violations include untrained employees, unlabeled hazardous chemicals and improperly inspected workspaces.

First Marine was cited for 24 violations and fined nearly $640,000. First Marine Vice President of Operations, Capt. Jason Straight, says the company’s analyzing its safety procedures.