(CNN) -- Underground teams exploring a West Virginia mine where 29 men died in an April explosion have found a crack in the floor that may have allowed methane to leak before the blast, mine owner Massey Energy confirmed Tuesday.

"The teams were not able to explore the crack to determine if it was the source of a methane explosion," a company statement said. "The crack and other potential sources in the mine need to be fully examined before any conclusions can be reached."

The explosion at the Upper Big Branch mine, about 30 miles south of Charleston, was the industry's worst disaster in 40 years.

The coal giant also released a July 2004 memo the company received from the Mine Safety and Health Administration regarding incident of methane "outbursts" at the mine, saying it shows the federal agency was inconsistent in its recommendations for air flow at the site.

"The July 15, 2004 report is very important," said Massey CEO Don Blankenship in Tuesday's statement. "MSHA recognized in 2004 that more airflow was necessary to address methane outbursts, should they occur. However, in 2009 and 2010, MSHA required UBB to adopt a ventilation system that resulted in less air."

MSHA responded in a statement Tuesday that investigators are exploring all of the circumstances surrounding the April 5 explosion.

"The investigation team is reviewing a large number of records including the 2004 memo Massey referenced, as well as inspection records and a significant number of more recent citations issued by MSHA for ventilation problems in Massey's mine in the months leading up to the explosion," the statement said.

"While the air quality in the sections of the mine being worked six years ago may provide some insight, the team's primary focus is on the circumstances occurring in the minutes, hours and months leading up to the explosion," it said.

Neither Massey nor MSHA provided documentation of the more recent requirements cited in Blankenship's statement.

Blankenship cautioned that "it is too early to determine what role these changes played or to determine the importance of the crack in the mine floor. However, this will be a focal point of our investigation and, we hope, the state and federal investigations."