NEW : Crews to pump nitrogen in West Virginia coal mine to flush out toxic gas . The company says it will use a new method of pumping the gas, which is not harmful to humans or animals, and that it has been approved by federal regulators for use at other mines. Cause of Monday 's explosion unknown ; state and federal officials plan full investigation .
NEW : Crews to pump nitrogen in West Virginia coal mine to flush out toxic gas . Rescuers hold onto hope of finding four miners trapped in blast . Cause of Monday 's explosion unknown ; state and federal officials plan full investigation .