CHARLESTON, W.Va. -- Federal prosecutors conducting a criminal investigation of Massey Energy's Upper Big Branch Mine are examining hundreds of violations, including some that occurred four years ago at the Raleigh County mine where 29 workers died in an explosion last month.

U.S. Attorney Chuck Miller confirmed the criminal inquiry - already widely reported in the news media for two weeks - in a letter Friday to lawyers for the Department of Labor's Mine Safety and Health Administration.

The letter, also signed by Assistant U.S. Attorney Booth Goodwin, revealed that prosecutors are investigating potential charges related to violations that have been under appeal by Massey's Performance Coal Co. for years.

"A portion of the criminal investigation involves whether Performance Coal and its directors, officers and agents engaged in willful criminal activity at UBB," said the letter, sent to Douglas N. White, associate regional solicitor for the Labor Department.

In an attachment to the letter, Miller and Goodwin listed 44 cases that Performance has pending before the Federal Mine Safety and Health Review Commission. They wrote to MSHA to ask the agency to seek a stay of the Massey appeals so that hearings on those civil cases do not interfere with the criminal investigation.

Government officials said the pending cases involve more than 500 citations and orders issued by MSHA between June 3, 2006, and the April 5, 2010, explosion for various safety violations, including inadequate roof support, ventilation problems and accumulations of combustible materials.

About 300 of the violations were classified by MSHA inspectors as significant and substantial, and more than 90 of them were alleged to be caused by the company's "high negligence or reckless disregard" for safety standards, officials said.

Mine operators can be prosecuted criminally for willfully violating health and safety standards, and mine managers can be prosecuted if they knowingly authorize such violations.

The letter was made public a day after an unrelated disclosure in U.S. District Court that MSHA investigators had found that a page was removed from a book where Performance Coal officials were required to record daily ventilation fan measurements at the Upper Big Branch Mine.

It is not clear if any daily measurements were removed or if the fan examination records were otherwise altered, but MSHA is investigating the matter.

"We do have a page that was ripped out," Kevin Stricklin, MSHA's coal administrator, said Friday. "It's something we're going to look into."

Stricklin said the fan examination book included entries for Sept. 29 and Sept. 30, 2009, but investigators found that a page between those two entries had been ripped out of the book.

"It may be a problem," Stricklin said. "We're going to look into it. It's not as big a concern to me as if the pre-shift examination was missing on the longwall section for two days prior to the explosion."

The missing page in the mine's fan examination book was first disclosed in a court filing by lawyers for the families of two of the miners killed in the Raleigh County disaster.

Mark and Rachel Moreland are suing MSHA to try to force the agency to hold the investigation's witness interviews in a public hearing, rather than behind closed doors.

In a legal brief, the Morelands said MSHA officials told miners' families about the missing page during a private meeting on May 5 at Liberty High School in Beckley.

"After much prodding by miners' families," the court brief said, "MSHA admitted that the investigative team determined that one page of a fireboss book had been removed."

The Morelands said "this disturbing example of mutilation of documentary evidence compels the conclusion that, in order to assure evidence is fully produced, and produced unaltered, MSHA must issue subpoenas," something the agency can only do if it calls a public hearing.

Shane Harvey, general counsel for Massey, said Thursday night he did not know what the Morelands were referring to in their court filing.

"I can say that we are cooperating fully in the investigation and have made and will make every effort to preserve and turn over all official mine records to the investigating agencies," Harvey said. "We will not tolerate any effort to alter or hide such evidence."

Massey had called previous news reports about a criminal inquiry at Upper Big Branch "unsubstantiated rumors," and said the company had no knowledge of any criminal wrongdoing.

However, in a sworn affidavit filed in federal court, Mark Moreland said he discussed rumors about a criminal inquiry with MSHA chief Joe Main before the May 5 meeting between MSHA and the disaster victims' families.

"I advised him that it was my understanding that it had been reported that 'another federal agency' had conducted in excess of two-dozen interviews in the community," Moreland said. "Mr. Main responded that he was sure that they likely had conducted far more interviews than that by the time of our conversation."

MSHA records also indicate that, in April and June of 2009, agency inspectors launched two "special investigations," a type of inquiry that frequently is the starting point for a criminal mine-safety prosecution. MSHA has declined to provide details of those special investigations.

Mine safety experts believe the April 5 explosion was caused by the ignition of methane gas and made far worse by the likely buildup of highly explosive coal dust in the underground mine.

So far, government investigators have not been able to get back into the mine to begin their on-site investigation. MSHA has begun continuous monitoring of the mine atmosphere to try to confirm whether the air quality is trending better or worse, after receiving reports from Massey that contradicted some previous readings.

Reach Ken Ward Jr. at kw...@wvgazette.com or 304-348-1702.