MARION, Ind. (AP) — Employees returned to work Wednesday at a General Motors metal-stamping plant where an explosion killed a contractor and injured several others a day earlier.

First-shift employees reported for work Wednesday morning at the plant in Marion, about 60 miles northeast of Indianapolis, GM spokeswoman Stephanie Jentgen said.

The plant was evacuated following Tuesday’s explosion that killed 48-year-old contractor James L. Gibson. An autopsy Wednesday showed he died of blunt force trauma to the head due to multiple skull fractures, Deputy Coroner Chris Butche told the Marion Chronicle-Tribune.

Marion Fire Chief Paul David said the explosion involved a chlorine dioxide tank. Jentgen said it isn’t known whether the chemicals in the area were involved with the explosion or simply in the area where the explosion occurred.

Robert Santini, a visiting scholar in chemistry at Purdue University, said chlorine dioxide is usually used to treat water. He said it is safe in low concentrations when kept cool in water, but can become volatile in a gaseous phase and becomes more concentrated.

Jentgen said four workers injured in the blast were held for observation at a local hospital but have been released. She says an investigation into what caused the explosion is underway.

Indiana Occupational Safety and Health Administration investigators have begun an investigation, said Chetrice Mosley, a spokeswoman for the state Department of Labor. She said such investigations usually take two to four months.

United Auto Works and GM health and safety professionals also will investigate the explosion, Jentgen said. It isn’t known how long the investigation will take.

The federal Occupational Safety and Health Administration Web site indicates eight complaints have been filed against the plant since 2004, three leading to findings of violations.

The plant was fined $845 by IOSHA in 2009 for a serious safety violation for having areas of machinery that were not guarded to prevent workers from having part of their bodies in danger zones. Two violations were listed in 2004, but one didn’t include a fine and the other said an informal settlement had been reached. Mosley said details weren’t available because the department doesn’t keep records that long.

The plant employs about 1,600 workers and provides blanks, stampings and sheet metal assembly for vehicles to GM assembly plants across North America. The explosion wasn’t expected to cause delays in supplying other plants, Jentgen said.

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