HENDERSON, Ky. (AP) — A western Kentucky plastics company that operates a plant where four people were injured in a flash fire had previously been fined $26,650 by state labor officials for safety violations deemed serious, Kentucky officials said Thursday.

The Kentucky Public Protection Cabinet released a series of citations issued to Polymer Partners LLC in Henderson between 2004 and 2008. The citations covered a range of violations, including using power-operated industrial trucks in atmospheres containing hazardous concentrations of explosive dust.

Wednesday's fire at the plant injured four people. William Swope with the Kentucky Fire Marshal's Office said two of the injured people were improving. The Occupational Safety and Health Administration is conducting a probe, he added.

Henderson Fire Chief Danny L. Froelich on Thursday said the cause of the fire remains under investigation.

The citations, issued by the Kentucky Labor Cabinet, include an Oct. 5, 2004 report of an explosion due to the presence of carbon black dust along with equipment malfunctioning as well as power-operated trucks being used in an atmosphere containing hazardous concentrations of dust. The company was fined $7,500 for those violations.

In November 2007, the Labor Cabinet penalized Polymer Partners $16,500 for a series of safety violations, including having unguarded electrical equipment and having an untrained employee conduct maintenance on energy controls.

The Labor Cabinet in January 2008 also cited the company for not keeping the production area clean of combustible dust.

Swope didn't release the names of the four injured Wednesday, but said the person taken to Vanderbilt University Medical Center in Nashville "appears to be the most seriously injured." The two people taken to a Louisville appear to be improving and the person attended to in Henderson was the least injured.

In response to questions about the fire, the company referred to a statement issued Wednesday. Company President Blaise Codispoti said in that statement that Polymer Partners was reaching out to family members of the injured workers.

Froelich said the accident happened in the manufacturing area of the facility, and fire crews extinguished a minor fire near a machine.

Polymer Partners is a plastics compounding manufacturer.