CHARLESTON, W.Va. -- Another West Virginia coal miner died Friday in an underground accident, just three weeks after 29 workers were killed in a huge explosion at a Massey Energy Mine in Raleigh County. The latest death came after a 28-year-old miner was crushed between a continuous mining machine and the mine wall late Thursday night at International Coal Group's Beckley Pocahontas Mine near Eccles. Amy Louviere, spokeswoman for the U.S. Mine Safety and Health Administration, said the miner -- whose name has not yet been released -- died Friday morning while undergoing surgery in Charleston. Late Friday, Gov. Joe Manchin again ordered the state's flags to half-staff on Sunday for the memorial service in Beckley to honor the miners killed and injured in the April 5 explosion at Massey's Upper Big Branch Mine near Montcoal. President Barack Obama and Vice President Joe Biden are both scheduled to attend and speak at the memorial, being held at the Beckley-Raleigh County Convention Center. Mine safety experts believe, based on preliminary information, that the explosion at Upper Big Branch involved an ignition of methane gas that was made far worse by a buildup of explosive coal dust underground. The mine had been repeatedly cited previously for inadequate ventilation and improper control of coal dust. MSHA officials have repeatedly said in recent weeks that such explosions are preventable if mine operators take proper precautions, such as following approved mine ventilation plans and spreading crushed limestone, or "rockdust," to control the buildup of coal dust underground. On Friday, Richmond, Va.-based Massey continued to defend itself against new lawsuits and national media reports that harshly criticized the company's safety practices. "Clearly, something went wrong at Upper Big Branch," said a statement issued by Massey spokesman Troy Andes. "But we simply don't yet know what it was. "If there was improper conduct regarding operations and safety, there will be accountability. "Serious accidents that lead to loss of life in any industry are typically not the result of one easily identifiable cause," the statement said. "Instead, they usually result from a complex interplay of contributing forces. Separating those strands of forces can take time and intense, expert study. That is why it is so important that for this accident, as for others, there be no rush to judgment about its cause." Investigators working to re-enter the Upper Big Branch Mine to begin the underground portion of their probe have been hampered by air readings showing gases that could indicate fires in the mine. Federal and state officials have said publicly that it will be at least a month before they can clear the mine atmosphere to make it safe for investigative teams, and have said privately it could be much longer -- up to three months or more. Experts say such delays are not especially unusual, given the size and scope of the explosion at Upper Big Branch and the sprawling nature of the mine's maze of underground tunnels and sealed, mined-out areas. "The main focus is the safety of our investigating teams," said Jama Jarrett, spokeswoman for the state Office of Miners' Health, Safety and Training. "We want to make sure the mine is safe before sending our teams underground and will work with MSHA and the company to determine the best plan and solution for reducing and eliminating the harmful gases currently being detected." Investigators had scheduled interviews to start Tuesday with mine rescue teams and with state and federal officials who inspected Upper Big Branch prior to the explosion. But the Obama administration has not yet responded to requests from two Upper Big Branch widows that the federal investigation of the disaster take place in a public hearing, instead of behind closed doors. Lawyers for Marlene Griffith, whose husband, William Griffith, was killed in the explosion, repeated their public hearing request on Friday in another letter to MSHA chief Joe Main, after Main did not respond to their initial request submitted earlier in the week. "Family members deserve and demand full transparency and a voice as they go through what is undoubtedly the most difficult time of their lives," the letter to Main said. Friday's death of an ICG miner in Raleigh County marked the 33rd U.S. coal-mining death in 2010. ICG's Beckley Pocahontas operation produced about 750,000 tons of coal last year with 200 workers, according to MSHA records. In 2008 and 2009, the mine recorded an accident rate worse than the national average. U.S. House Labor Committee Chairman George Miller, D-Calif., said Friday that the mine was among 48 coal operations that avoided tougher enforcement action by MSHA by repeatedly challenging citations and orders issued by federal inspectors. "Indiscriminate mine owner appeals are letting some of the most dangerous mines escape tougher penalties and heightened scrutiny," Miller said. "The Obama administration and Congress must correct this problem and correct it now." Ira Gamm, spokesman for Scott Depot-based ICG, declined to comment on the Beckley Pocahontas mine's safety record. Reach Ken Ward Jr. at firstname.lastname@example.org or 304-348-1702.