The US Occupational Safety and Health Administration (OSHA) has announced an enforcement action against DuPont in connection with an incident that killed four workers at its La Porte, Texas, plant in November 2014.

OSHA also identified a number of unspecified safety upgrades DuPont must implement to prevent future accidents.

The workers who died were exposed to methyl mercaptan gas when opening a drain on a vent line, the investigation showed.

DuPont was cited for 11 violations, one repeat violation, nine serious violations and one "other than serious" violation. The repeat violation pertained to the lack of training for employees on using the building's ventilation system and other safety procedures - for which the company was also cited in July 2010.

The agency has proposed penalties totaling $99,000.

The La Porte accident, one in a series of serious and less serious mishaps at DuPont's US plants in the recent past, according to news reports, was also being reviewed by the US Chemical Safety Board.