Rescue efforts have resumed at the West Virginia’s Upper Big Branch coal mine where twenty-five miners were killed in an explosion earlier this week. Search teams are seeking four missing miners in the hopes they managed to escape to an underground safety chamber. The rescue effort has faced repeated delays in order to filter out dangerous gases. On Thursday, West Virginia Governor Joe Manchin said the delays were needed to protect the rescue team.

Gov. Joe Manchin: “The quality of the air since this morning, since the men have gone in, the quality of the air has deteriorated. We were concerned about what conditions that air would cause with an explosive atmosphere.”

More details continue to emerge about safety violations at the Upper Big Branch mine before Monday’s explosion. A Senate report says federal regulators have issued evacuation orders at the mine more than sixty times since the start of 2009. In 2007, the mine’s owner and operator, Massey Energy, was able to prevent the Mine Safety and Health Administration — MSHA — from declaring a pattern of violations, which could have led to the mine’s closure. According to MSHA, the mine exceeded national averages in eleven safety citation categories. For the most serious citation categories, the mine had more than eleven times the national rate.