Investigations are in progress into alleged maintenance lapses and tampering of equipment that monitors methane levels at the Massey Energy mine in the US.

An explosion at the mine killed 29 workers in April.

Massey’s Upper Big Branch Mine was cited 17 times by federal officials for failing to keep methane monitors in working condition, according to The Wall Street Journal.

Federal safety officials will test two methane monitoring systems found near the place of the explosion on 7 August for possible indications that the devices were functioning or disabled.

Though a build-up of methane is believed to be the likely cause of the explosion, a definitive conclusion is yet to be reached.