ConAgra declined to discuss the 26 citations and $134,773 in fines leveled by safety inspectors.
"Since the accident, we have worked closely and fully cooperated with the agency throughout its investigation. We are reviewing their comments and findings, but cannot comment on them at this time," the company said in a statement.
Energy Systems Analysts Inc. was cited for 28 serious violations and fined $58,100. A message left at the company's offices after business hours was not returned Tuesday.
An Energy Systems employee last month became the fourth fatality resulting from the June 9 blast.
More than 200 people were working in the plant when the explosion caused part of the plant's roof to collapse. Three ConAgra workers were killed when they were struck by debris or crushed when part of the building collapsed.
The state agency's investigation offered more details into what caused the blast.
For about three hours on the day of the explosion, a contractor working in a pump room was trying to light a natural gas-fired water heater. The outside employee, in the presence of ConAgra management, improperly allowed gas to escape into the enclosed room that contained electric motors and circuit panels, which could have ignited the blast, the state agency said.
Two federal agencies had previously blamed the explosion on natural gas. The Chemical Safety Board said contractors installing the water heater likely vented natural gas inside the building before the explosion as they purged a gas line. Officials said the gas should have been vented outside.
The Labor Department also opened inspections of 14 other companies after the blast, but none were cited for workplace violations.
ConAgra and Energy Systems have 15 working days to contest the citations or pay the penalties.