( CNN ) -- The West Virginia coal mine where 29 miners died in an explosion in April had a sudden and large inundation of methane gas the day of the accident , the mine owner said Thursday . The level of methane -- an explosive gas -- was almost three times greater than normal , Massey Energy Company said . It is still too early to determine what caused the April 5 explosion , but the methane gas data is a `` very important piece of evidence '' in the ongoing investigation , said Christopher Schemel , a leading explosions expert and a member of the investigation team . Investigators tested data from the mine 's primary ventilation exhaust fan and found the unexpected release of gas was intense and overwhelmed the normal safety systems , the company said . `` To put it in perspective , a methane release of this size would completely fill a 2,000 square-foot house with an explosive atmosphere in under 40 seconds , and could fill the volume of a typical mine entry to explosive levels in under 25 seconds , '' Schemel said . The explosion at the Upper Big Branch mine in Naoma , about 30 miles south of Charleston , was the industry 's worst disaster in 40 years . The investigation since then has focused on the mine 's poor safety record and Massey 's response to problems in the past . A preliminary report on the disaster by the federal Mine Safety and Health Administration found that the Upper Big Branch mine had `` a significant history of safety issues '' and that the agency was closely monitoring Massey Energy . In 2009 , federal inspectors issued 515 citations and orders at the Upper Big Branch mine , with 39 percent of them for `` significant and substantial '' violations . More than 50 of the citations were for problems that the operators knew about but had not corrected , according to federal mine safety records . Thirty-one men were working in the mine when the explosion happened on a Monday afternoon . Most of the blast victims were working in an area where long wall cutting was taking place . The technique uses a large grinder to extract the coal and creates large amounts of coal dust and methane gas , both of which are explosive . Recovery crews found some of the bodies in a section of the mine where readings showed high levels of gases , and the crews needed breathing gear to enter , said Jama Jarrett , spokeswoman for the West Virginia Office of Miners ' Health , Safety and Training . Rescuers had to use high-pressure fans to pull the toxic air through holes to the surface . CNN 's Melissa Gray contributed to this report .
2008 : The explosion was the mining industry 's worst disaster in 40 years . The West Virginia mine that exploded in 2008 had a sudden influx of gas . It 's too soon to say whether the gas caused the explosion , or say . The amount of methane was almost three times greater than normal .