The U.S. Occupational Safety and Health Administration opened its investigation Tuesday, shortly after the fire started, spokeswoman Elizabeth Todd said. Details of the probe will be made public once it is complete, she added.
The Texas Commission on Environmental Quality will also investigate whether the plant violated air emissions standards due to the accident.
Enterprise spokesman Rick Rainey has said the company is also probing the cause of the accident, including possible pipeline failure.
About one-third of the nation's oil is refined along the Texas Gulf Coast, an area covered with massive plants, mazes of pipes and towering industrial stacks. The economy is reliant on the oil and gas industry, and fires, accidents and explosions are not uncommon. Residents sometimes joke that the sweet stench of petroleum is the smell of money and are accustomed to "shelter in place" orders, when they are forced to rush inside and shut all their windows and doors until a danger -- including toxic chemical releases -- passes.
In September, two workers at BP's Texas City refinery -- the site of a 2005 explosion that killed 15 workers and injured 170 others -- were rushed to the hospital with steam burns. The U.S. Environmental Protection Agency is investigating the facility for an apparent 41-day benzene release that coincided with BP's involvement in the Gulf of Mexico oil spill. In May, thick, black smoke could be seen for miles when a fire erupted at a 700-acre facility owned by LyondellBasell Industries. No one was injured in that incident.