The Jackson (Miss.) Clarion-Ledger said Friday that federal and private investigators were attempting to determine what caused the pipeline blast during a pressure test by Grand Bluff Construction Co.
"We honestly don't know," an unidentified official with Grand Bluff, a third-party contractor conducting tests at the site, said.
A spokesman for Kinder Morgan, which owns the pipeline from Wednesday's blast, said pressure tests involving nitrogen gas are routine ways to determine a pipeline's integrity.
"Pipeline tests are often pressured to and above the maximum operating pressure to ensure the integrity of the line before it is put into service," spokesman Joe Hollier said.
The Clarion-Ledger said 40-year-old worker James Lee Candler was killed in the explosion, while three other individuals were injured.
The injured workers, identified as Rick Lee, 50; Wayne Williams, 31; and David Matthews, 51; are listed in good, fair and serious condition respectively.