CHARLESTON, W.Va. -- Federal prosecutors say their investigation of the Upper Big Branch Mine Disaster has entered a vital phase, with new information giving them key leads toward more potential criminal charges.Assistant U.S. Attorney Steve Ruby revealed the information in a new court filing that seeks to delay certain proceedings in a civil case by former Massey Energy shareholders who say they were deliberately misled about the company's safety record before the April 2010 explosion that killed 29 miners."The United States' criminal investigation of circumstances relating to the Upper Big Branch Mine is in a critical period, developing valuable information that became available only recently," the court filing said.The government revealed no additional details in a court document filed as part of efforts to resolve a dispute over whether lawyers for the former Massey shareholders can conduct legal discovery -- obtaining documents and conducting interviews -- without jeopardizing the criminal probe.Prosecutors and the former Massey company and executives agreed to ask U.S. District Judge Irene Berger to delay discovery in the shareholder case until January, the same deadline set in a similar case in Delaware business court.In the court filing, the government said the delay would "allow the United States to pursue those leads and make decisions on potential charges without interference from civil discovery."Prosecutors said that allowing discovery would give defendants in the civil case -- including most former top Massey executives -- access to documents they don't currently have about the company's operations at Upper Big Branch. Some of those defendants "may be or may become subjects of the criminal investigation," the government said, and access to such records would "allow them to anticipate in detail the direction of the criminal investigation.""Civil discovery also would allow the civil parties to shape the record extensively for their own ends, making the United States' investigation -- and possible prosecutions -- more difficult," the court filing said.Prosecutors also said that "even limited civil discovery" in the case would divert time and effort from the criminal investigation.Lawyers for the former Massey shareholders argue that the government and the defendants are seeking too strict of a delay, and that they should be allowed to move forward sooner in some discovery activities.Last month, U.S. Attorney Booth Goodwin sought and obtained a delay in the sentencing for former UBB mine superintendent Gary May, who has pleaded guilty and is cooperating with investigators. Goodwin argued that delay was needed while prosecutors used information May has provided to further their probe.May admitted that he plotted "with others known and unknown" to put coal production ahead of worker safety and to conceal the resulting hazards on numerous occasions at Upper Big Branch.May, 43, of Bloomingrose, admitted that he took part in a scheme to provide advance warning of government inspections and then hide or correct violations before federal agents could make it into working sections of the sprawling mine.So far, May is the third person to be charged in the sprawling federal criminal investigation at Upper Big Branch.Thomas Harrah, a former miner at the site, was sentenced to 10 months in jail after he admitted to faking a foreman's license when he performed key mine safety examinations at the mine between January 2008 and August 2009 and to then lying to investigators about his actions.Former Massey Energy security director Hughie Elbert Stover is appealing a three-year jail sentence he received after being convicted of lying to investigators and trying to destroy evidence about Massey's practice of warning underground workers of impending government inspections.In December, Goodwin and his team secured a $209.5 million settlement with Alpha Natural Resources, which acquired the Upper Big Branch Mine when it bought Massey Energy.Goodwin agreed not to prosecute the company for any Upper Big Branch criminal liabilities, but required Alpha to spend $80 million over the next two years on mine safety improvements and create a $48 million mine safety research trust fund. Alpha also agreed to pay $46.5 million in restitution to families of the disaster victims and $35 million to resolve pending Massey safety fines, including $10.8 million levied for violations related to the Upper Big Branch explosion.The settlement with Alpha, however, did not prohibit prosecutors from pursuing charges against any individuals -- including Massey officers, employees or agents -- who played a role in the mine disaster.Reach Ken Ward Jr. at email@example.com or 304-348-1702.