Federal investigators say a natural gas leak ignited by electrical equipment caused an explosion at a Slim Jim plant in North Carolina that killed three workers.

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U.S. Bureau of Alcohol, Tobacco, Fireams and Explosives agent Earl Woodham said Saturday his agency has turned the case over to workplace investigators to determine the cause of the leak and what piece of machinery sparked the blast.

The explosion Wednesday ripped through the 500,000-square-foot ConAgra Foods Inc. plant in Garner while 300 people were at work, tearing a hole in the roof.

Officials said 38 employees were injured, four of them with critical burns.

Woodham says ATF agents consider the blast an industrial accident and have closed their criminal investigation.