detectors meant to protect workers from explosive methane gas may have been disabled intentionally. here's nbc's tom costello. >> reporter: it was february 13th, two months before 29 miners died at the upper big branch mine in west virginia when something happened at the mine that scared ricky lee campbell. what authorities say could have been criminal tamp weering with methane gas detector in a mine where explosive gas has emerged as a real concern. >> i witnessed a electrician bypassing a methane detector on the machines. >> reporter: the machines that cut coal? >> yes, sir. >> reporter: why would they do that? >> so we can continue to run coal. >> reporter: who's not clear if the gas monitor was detecting methane or instead malfunctioning and shutting down the machinery. but campbell says he's told the fbi that a supervisor ordered the electrician to disable it.