The Bureau of Alcohol, Tobacco and Firearms determined the blast to be "an unfortunate industrial accident" with no signs of criminal intent.
Two explosions at the facility at about 8 p.m. on December 28 damaged the facility and rocked surrounding homes and businesses.
Fourteen people were taken to local hospitals for treatment. The explosions and fire are believed to have started near an oil-fueled furnace used to melt aluminum.
In July, Yaffe received a notice of several severe violations of OSHA regulations after an inspection conducted in May, reported the Muskogee Phoenix. But all violations cited had been corrected after the company was issued the citations, according to OSHA.
The company was cited for substituting compressed oxygen for compressed air to dust-off containers and blowing debris out of tools with no safety relief devices on the tanks.
The company also was cited for slings and fastenings and attachments not being inspected for defects by qualified personnel, and that employees were exposed to "struck-by" hazards from 1,500-lb. aluminum casings supported by a damaged rope sling.
Also, the company was cited by OSHA for using electrical equipment not approved for use in an industrial environment, and an electrical outlet box near the furnace did not have effective covers to prevent electrical shock.
The company was fined $4,965 by OSHA for the violations, and the issues were resolved by August, according to OSHA documentation.