Federal and safety mine officials have been evacuated from the Massey-owned Upper Big Branch coal mine in the US due to high levels of methane.

Mine Safety and Health Administration spokesperson Amy Louviere said methane levels of 2-3% were detected from a monitoring hole connected to the surface of Massey’s Upper Big Branch Mine in Montcoal.

The methane was detected underground near the mine’s longwall, Louviere added, reports The Wall Street Journal.

The investigation into the blast, which killed 29 miners in April, has been hampered by high levels of carbon monoxide, methane and water accumulations.