The U.S. Occupational Safety and Health Administration fined a Geismar biodiesel plant $70,000 over a Sept. 3 hydrogen gas explosion and fire that injured four workers, including at least one who was severely burned.

The Renewable Energy Group, a biofuels producer headquartered in Iowa, was also cited with three “willful” safety violations — OSHA’s most severe category of violation — over the company’s failure to ensure a hazardous and flammable chemical was no longer in a plant pipeline that was under repair, according to a citation signed Thursday by OSHA Area Director Dorinda Folse.

A willful violation means the company demonstrated “intentional disregard” or “plain indifference” to employee safety and health, the OSHA field operations manual says.

In the citation, OSHA gave REG through March 18 to correct the violations.

REG officials said they have been cooperating with state and federal authorities looking into the explosion but plan to contest OSHA’s fine and the citation.

“While we have been working with OSHA throughout its investigative process, we respectfully disagree with its findings and the classification of the citation and will contest it,” Anthony Hulen, REG spokesman, said in a statement Saturday. “We expect to meet with OSHA in the coming weeks to discuss its findings, the safety measures already in place at the Geismar facility and our commitment to continuous improvement.”

Hulen added REG’s first priority is the safety of its workers and contractors and the Geismar community.

OSHA officials were not available for comment over the weekend.

A State Police investigation, completed independently of the federal probe into the September explosion, found REG workers failed to take recommended safety steps to ensure highly flammable hydrogen gas was no longer flowing from a storage tank into a high-pressure line that was being repaired.

The hydrogen gas, which is invisible and odorless, escaped when REG workers and Excel Group contractors took apart valves in the line for upgrades after they believed them to be free of gas. The gas, continuing to flow out, caught fire and exploded, troopers said.

As in the State Police investigation, which was completed earlier this year, the OSHA citation revolved around alleged failures in so-called “lockout/tagout” procedures.

These are steps designed to ensure a valve or other equipment has been locked — often with a padlock — in the closed position and is tagged, or clearly marked, so other workers know it’s closed.

In the citation, OSHA found REG failed to develop safe work practices related to this lockout/tagout process, did not ensure those procedures were written down clearly and did not ensure workers had an effective process to verify when valves were locked out.

In the State Police investigation, troopers reported that an REG lawyer acknowledged a key valve intended to shut off the gas flow had been left open.

The explosion was the second at REG’s Geismar facility in 2015. State Police cited REG with careless handling of a hazardous material for the Sept. 3 explosion but did not fine the company.

State Police did not cite REG for an explosion April 2, which was caused by a leaking solvent pump that had been repaired.

The status of a pending OSHA investigation into the April 2 explosion was not clear Sunday.

Follow David J. Mitchell on Twitter, @NewsieDave.