GARNER (North Carolina) - AN explosion that killed three workers at a snack foods plant earlier this week was caused by a natural gas leak that ignited in a room housing vacuum pumps for sealing the snacks, authorities said on 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 US 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.

The explosion ripped through the 500,000-square-foot (46,500-square-meter) plant while 300 people were at work. Officials said 38 employees were injured, four of them suffering critical burns. Three firefighters were treated after inhaling fumes from ammonia, which is used in the plant as a refrigerant. -- AP