According to an Associated Press report, the meter detected 5% methane in the atmosphere of the Raleigh County operation where 29 workers died.

The gas is considered to be at an explosive level when readings underground reflect 5-15% methane.

US Mine Safety and Health Administration coal administrator Kevin Stricklin told the AP the unit was located underground near the mine’s longwall.

Six of the deceased were discovered in that location.

The agency has been investigating the explosion since it occurred in April, though the evidence collection portion of the review could not begin until the mine was cleared for entry in late June by a special team that toured the entire operation.

Neither Massey officials nor a spokesperson from MSHA immediately responded to an ILN request for comment, and no public statements in response to the AP report were released Friday.

In a preliminary report authored by MSHA investigators and presented to US President Barack Obama earlier this year, the agency cited methane and coal dust as a potential cause of the UBB blast.

Its full investigation is ongoing, with no completion timeline set.

UBB mine owner Massey has also looked to methane as a potential cause, though it said high levels of the gas may have poured into the operation just prior to the explosion, overwhelming its installed safeguards.

MSHA and Massey have been at odds recently over the possibility that UBB crews may have electronically “bridged” machine-mounted monitors to keep them from powering down a unit when methane levels became hazardous.

Massey has denied allegations of the practice, which MSHA said was routine according to its interviews with former employees.

Stricklin told the AP investigators will search again for more monitors, which could be buried beneath the rubble of the blast.

"We expected to find at least two remotes and we only got one," he told the news service.

"The one detector that we found was the one detector that had seen 5 percent of gas."

In related UBB explosion news, Massey has reportedly settled with four of the 29 families of the miners who died.

According to local news outlet West Virginia MetroNews, the families of Rick Lane, Timmy Davis Sr, Rex Mullins and James Mooney have all agreed to compensation agreements.

Terms of the settlements are sealed and were not made public.

The status of any pending settlements with the remaining 24 families is unknown, though one family did reportedly file a wrongful death lawsuit earlier this year.