U.S. Chemical Safety Board Chairman John Bresland said venting a line that contained natural gas inside a building would have been a highly unusual procedure.
"In my mind that would seem to be a risky operation," he offered.
In addition to the three workers killed, 38 employees and three firefighters were injured. Four of those employees suffered critical burns.
- Click here to read about the explosion and the investigation
- Read ConAgra's report filed with the Securities and Exchange Commission on June 17
- Click here to see photos of the ConAgra Foods plant on the day of the explosion
Two injured workers filed lawsuits earlier this week against the contracting firm hired by ConAgra to install the heater and a natural gas line to it.
There are ten federal investigators working on the review of what happened. Bresland said the work would likely take weeks. So far, they've interviewed more than a hundred eyewitnesses.