CHARLESTON, W.Va. -- Federal agents are interviewing current and former Massey Energy employees as part of a sprawling criminal investigation into the April 5 explosion that killed 29 miners at the company's Upper Big Branch Mine in Raleigh County, according to sources familiar with the inquiry. Officials from the U.S. Attorney's Office, the FBI and special investigators from the U.S. Mine Safety and Health Administration have been conducting the interviews. Federal and state sources discussed the interviews on the condition that they not be named, because they were not authorized to talk about the criminal investigation. U.S. Attorney Chuck Miller in Charleston referred all questions about the matter to Department of Justice officials in Washington, who declined official comment. Investigators are focused on finding out if any criminal violations of mandatory health and safety standards for underground coal mines were committed that might have been involved in causing the explosion. Such violations are misdemeanors, while any faking of required mine safety-check records or other safety documents required by MSHA is a felony. Massey issued a statement that referred to "unsubstantiated rumors" of a criminal investigation and said Massey "has no knowledge of criminal wrongdoing." "It is not uncommon that an accident of the size and scope of UBB would lead to a comprehensive investigation by relevant enforcement agencies," the Massey statement said. "We are cooperating with all agencies that are investigating the tragedy at UBB. Massey does not and will not tolerate any improper or illegal conduct and will respond aggressively as circumstances warrant." Investigators believe the huge explosion was caused by the ignition of methane and probably made far worse by accumulations of coal dust. Federal and state officials have delayed going back underground to gather evidence because sampling has found gases that could indicate there is an ongoing fire. Massey is pumping nitrogen into the mine to put out the fire, but officials have said privately it could be several months before it is safe for teams to re-enter the mine. Formal interviews for the civil investigation by MSHA, the state Office of Miners Health, Safety and Training -- as well as special state investigator Davitt McAteer -- also have been on hold for weeks. MSHA chief Joe Main is under increasing pressure, including full-page newspaper ads from his former employer, the United Mine Workers union, to conduct the entire investigation through a public hearing. Several news organizations, including The Charleston Gazette, also have asked for public access to the civil investigation. Mine safety experts said Friday it is not surprising or uncommon for criminal authorities to investigate a mine disaster of this size, but said it is far too early to know where the inquiry might lead. Major mining accidents causing multiple deaths in 1989, 1991 and 1992 all resulted in criminal charges against mine operators and company officials. The last four coal-mining disasters -- defined by regulators as five or more deaths in one incident -- did not produce criminal charges. Two weeks ago in a White House Rose Garden speech, however, President Obama said he had instructed Labor Secretary Hilda Solis and MSHA to "work with the Justice Department to ensure that every tool in the federal government is available in this investigation." Before that, in an April 12 statement, Miller had said his office was "ready, willing and able" to receive any information about possible criminal wrongdoing related to Upper Big Branch. Officials from Miller's office and the Justice Department said Friday they are prohibited by government policy from confirming or denying the existence of any criminal investigation. However, in April 2006, amid growing media speculation, Miller did issue a statement confirming that his office had initiated a criminal investigation of the January 2006 fire that killed miners Don Bragg and Ellery Hatfield at Massey's Aracoma Alma No. 1 Mine in Logan County. That investigation resulted in Massey's Aracoma Coal subsidiary pleading guilty to 10 criminal violations and paying a $2.5 million criminal fine, as well as admitting that one of the violations "resulted in the deaths" of Bragg and Hatfield. Still, the Bragg and Hatfield families opposed the plea bargain. They were upset that Miller's office had agreed not to try to prosecute the Massey parent company or any high-ranking corporate officers. Another Massey company, White Buck Coal Co., pleaded guilty three years ago and paid a $50,000 fine for not conducting required mine safety checks, and in 2003, two Massey subsidiaries pleaded guilty to criminal Clean Water Act violations. Staff writers Andrew Clevenger and Gary Harki contributed to this report. Reach Ken Ward Jr. at email@example.com or 304-348-1702.