Tests of methane monitors taken from Massey Energy’s Upper Big Branch mine in West Virginia, US, reveal that the devices had not been tampered with before the explosion that killed 29 miners in April.

State Office of Miners’ Health Safety and Training chief Ron Wooten said the tests show the monitors had not been tampered with electronically.

The methane monitor tests were conducted after former employees of Massey alleged the machines were electronically bridged to keep them running even if a high level of methane gas was detected.

Wooten added the tests, however, did not disprove allegations that bags were often put over the monitors’ sensors to alter their readings, reports Associated Press.

It may take up to next year before an official cause of the disaster is known, according to local reports.