North Carolina ? An explosion that killed three workers at a North Carolina Slim Jim plant was caused by a natural gas leak that ignited in a room housing vacuum pumps for sealing the snacks, authorities said Saturday.

It will now be up to state and federal workplace investigators to determine how the leak happened and what caused it to ignite in Tuesday’s blast at the ConAgra Foods Inc. plant in Garner, said Earl Woodham, an agent with the U.S. Bureau of Alcohol, Tobacco, Fireams and Explosives.

ATF agents believe the gas was sparked by a piece of equipment such as a fan motor or thermostat, but Woodham said another cause, like static electricity, couldn’t be ruled out. Such electrical equipment would be capable of catching natural gas on fire even if it were operating normally, he said.

The ATF concluded the explosion was an accident and closed its criminal investigation.