Read the Coal Tattoo blog for updates.CHARLESTON, W.Va. -- Unsafe gases and a possible fire are still keeping investigators out of the Massey Energy Co. mine where 29 workers died in a massive underground explosion on April 5, officials said Wednesday."Indications are that there is still some type of fire or heating underground, due to the continued presence of acetylene and ethylene," said Amy Louviere, a spokeswoman for the U.S. Mine Safety and Health Administration.Investigators had hoped to re-enter the Upper Big Branch Mine in Raleigh County this week. They have said publicly that it likely will be at least another month before that happens. Privately, they have said it could be three months or more.Louviere said officials continue to monitor the mine atmosphere and that when the investigation can begin underground is a day-to-day issue.Massey has pumped 7 million cubic feet of nitrogen into the mine in an effort to put out any fires, Louviere said. While the concentration of gases has "decreased somewhat," it's not enough to allow investigation teams to enter, she said.Investigators and mine safety experts believe the April 5 explosion likely was caused by an ignition of methane gas that was then made far worse by a buildup of explosive coal dust underground. MSHA and the state Office of Miners Health, Safety and Training are investigating. Additionally, Gov. Joe Manchin appointed longtime mine safety advocate Davitt McAteer to conduct an independent inquiry of the worst U.S. coal mine disaster in 40 years.Louviere said the MSHA investigators are reviewing mine records and putting together a list of witnesses to interview.Interviews had been scheduled to start this week, but have been delayed for at least another week.The United Mine Workers union and at least two victims' families have called for the interviews -- normally conducted behind closed doors -- to be held as a public hearing. On Tuesday, a coalition of media outlets and journalism organizations, including The Charleston Gazette, wrote to MSHA chief Joe Main to request news media and public access to the investigation.Louviere said MSHA does not have a timetable for when interviews will start or for when Main will respond to calls for a public hearing.Reach Ken Ward Jr. at firstname.lastname@example.org or 304-348-1702.