WASHINGTON -- Federal investigators are examining the possibility that officials at Upper Big Branch mine in West Virginia tampered with ventilation records before an explosion killed 29 miners there.

The federal Mine Safety and Health Administration confirmed Monday that a page is missing from the Upper Big Branch fan book, in which mine personnel log their daily checks of ventilation fans.

The mine reportedly had been cited for ventilation problems and MSHA officials had halted work there 61 times since the beginning of 2009 for safety concerns.

Though the investigation is in its infancy, the explosion is believed to have been caused by a combination of methane and coal dust buildup, and inadequate ventilation is a likely culprit.

The missing fan-book page comes between Sept. 30 and Oct. 1, several months before the April 5 explosion but just a week after MSHA inspectors issued three brief, partial-shutdown orders in a single day to remedy safety concerns. The citations did not include the fan system.

The MSHA is looking at whether the missing page is a simple error -- all the inspection days are accounted for at this point -- or a willful cover-up, though because of the timing it most likely had no impact on the explosion.

"At this point we're taking it seriously and looking at it," MSHA spokesman Amy Louviere wrote in an e-mail.

Massey Energy, which owns Upper Big Branch, downplayed the allegation.

A Massey spokeswoman wrote in an e-mail: "We've been told by MSHA representatives the missing page is not a focus of concern."

National Public Radio reported Monday night that the FBI criminal investigation has targeted the mine's methane detectors and ventilation plan.

NPR reported that FBI investigators have been asking miners whether a Massey supervisor made unauthorized changes to the mine's ventilation plan, and if they had ever disabled methane monitors.

The missing page in the fan book was first publicly disclosed in a federal lawsuit against the MSHA filed by two of the dead miners' families. The families are asking for an injunction to force the MSHA to open up all its investigation interviews to the public.

The MSHA will conduct several public hearings on the tragedy but has said it will keep interviews private in order to protect the anonymity of whistleblowers.

Attorney Mark Moreland, who represents the estates of miners William Griffith and Ronald Maynor, and other critics, including the United Mine Workers, who have joined the suit, claim that the privacy will keep a lid on any revelations that would damage the MSHA.

"At this point they control entirely any information that comes out of their investigation," Moreland said. "The bottom line is, MSHA's involvement here is sort of under the auspices of 'trust me,' and MSHA just doesn't carry a lot of trust in the mines or the community surrounding Upper Big Branch, at least as I've observed."

He pointed out that the missing page from the log book wasn't disclosed until the families pressed MSHA officials about it at a May 5 meeting.