A ground-rattling explosion Thursday at a chemical plant in Louisiana ignited a blaze that killed at least one person and injured dozens of others, authorities said. Witnesses described a chaotic scene of flames high in the air and workers scrambling over gates to escape the plant.

A thick plume of black smoke rose from the plant after the blast even after the fire was extinguished. At a roadblock several miles away where family members waited anxiously to hear about loved ones, flames were still easily visible above the trees even hours later.

Louisiana's health department said 77 people were treated at hospitals, with 51 being released by the evening. Hospitals reported that workers mostly had burns, cardiac and respiratory issues and bruises, health department spokeswoman Christina Stephens said in a news release.

A body was found by hazardous materials crews going through the aftermath of the blast at the facility, state police Capt. Doug Cain said. Trooper Jared Sandifer said the man killed was 29-year-old Zachary C. Green.

The company said the blast happened at 8:37 a.m.  By the afternoon, all of the plant's more than 300 workers had been accounted for, Cain said. The plant, owned by The Williams Companies Inc., based in Tulsa, Oklahoma, is in an industrial area of Geismar, a Mississippi River community about 20 miles (32 kilometers) southeast of Baton Rouge.

The Williams facility is one of scores of chemical and industrial facilities that dot the riverside between Baton Rouge and New Orleans. A few homes and four other plants are within 2 miles (3.2 kilometers), said Lester Kenyon, spokesman for Ascension Parish government.

The cause was not immediately known but the FBI said terrorism was not suspected.

A contract worker, Daniel Cuthbertson, 34, described a scene of “mass hysteria” immediately after the explosion, with workers scrambling over gates to get out of the plant.

More than 300 people were evacuated from the site, but some stayed behind, officials said. Ten workers stayed in an explosive-proof control center as the fire raged, Cain said. The workers performed vital tasks, including shutting valves that rendered the plant safe, he said.

Residents several miles from the plant described feeling the ground shaking.

“It felt like a three-second earthquake. It was a massive explosion,” said state Sen. Troy Brown, who lives several miles from the plant. Unsure what it was, he drove to a gas station down the street and saw flames shooting up to 200 feet (60 meters) into the air.

“It was scary,” he said.

Officials at area hospitals said a handful of patients were in critical or serious condition, though most seemed to have minor injuries. The plant makes ethylene and propylene _ highly flammable gases that are the basic building blocks in the petrochemical industry.

Early tests did not indicate dangerous levels of any chemicals around the plant after the blast, but Cain said air monitoring continued Thursday afternoon.

Cain said the fire was out, but gas was being flared _ burned at the top of high chimneys _ in other parts of the plant. “There is still some controlled flaring going on, so people in the area are going to see smoke,” he said. (AP)