"We've got two dead -- one confirmed dead and one missing presumed dead," Jean Kelly, a spokeswoman for the state's department of environmental quality, which is investigating the blast, told AFP.
Some 600 hundred workers were inside when the blast occurred just before 9:00 am (1830 IST) at the plant in Geismar, a small town north of New Orleans.
Nearby residents told local media that they could feel their homes shake from the blast and some saw a huge fireball engulf part of the sprawling plant.
"We've completed our first round of monitoring," Kelly said. "We didn't see any spikes of chemicals in the air."
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The shelter in place order was lifted for everyone except two other plants near the blast.
"The fire at Williams Olefin continues to burn but is under control at this time," the Iberville Parish Council said in a midday update on its Facebook page. "Please continue to monitor the situation until an all clear is given."
"We are in the process of accounting for all personnel," the company said in a statement.
"Our emergency-response crews are thoroughly trained to respond to these types of incidents and are diligently performing their work with their first priority being the safety and well-being of people in and around the area."
Louisiana Governor Bobby Jindal, who was on his way to the site to survey the damage, tweeted: "Praying for those affected by the Geismar plant explosion."
The facility produces approximately 1.3 billion pounds of ethylene and 90 million pounds of polymer grade propylene a year, according to the company's website.