(AP) - Federal prosecutors investigating the West Virginia coal mine explosion that killed 29 men are working their way up the corporate ladder with criminal charges. On Wednesday, the former superintendent of the Upper Big Branch mine became the highest-ranking company official charged in the 2010 disaster, and he is apparently cooperating with prosecutors, who said the investigation is far from over. Gary May, 43, was charged with conspiracy to defraud the government, accused among other things of disabling a methane gas monitor, falsifying safety records and using code words to tip off miners underground about surprise inspections. He could get up to five years in prison if found guilty. The only other Massey Energy employee prosecuted so far in the nation's deadliest mining disaster in four decades is former mine security chief Hughie Elbert Stover, who will be sentenced next week for lying to investigators and trying to destroy documents. Prosecutors are urging a federal judge to make an example of Stover by giving him the maximum - 25 years in prison. The charges against May were contained in what is known as a federal information, a document that typically signals a defendant is cooperating with prosecutors. Reached at his home Wednesday, May declined to comment. U.S. Attorney Booth Goodwin said his investigation is "absolutely not" finished, but he would not comment further. Prosecutors routinely cut deals with lower-ranking figures to help them build cases against higher-ranking ones. Clay Mullins, who worked at Upper Big Branch and lost his brother Rex in the blast, said more people deserve to be prosecuted, including former Massey CEO Don Blankenship. "I want to see some other names," he said. There were a lot of people involved in this, and I just want to see them be punished for the crimes. Prosecutors have refused to say whether they are targeting Blankenship, a hard-nosed executive whose company was cited for violations so frequently that union critics accused him of regarding fines as simply the cost of doing business. Blankenship, once one of the most outspoken leaders in the coal industry, retired months after the explosion and moved away from West Virginia. A telephone number for him could not be found, and he has all but disappeared from public view since the blast. A United Mine Workers report said Blankenship was among 18 Massey employees who invoked their right against self-incrimination and refused to cooperate with any of the investigations. Alpha Natural Resources of Bristol, Va., bought Massey and all its operations, including the Upper Big Branch mine, last summer. Prosecutors have accused Massey of violating a host of safety laws out of a desire to put production and profits first. Three investigations of the tragedy concluded that the company allowed highly explosive methane and coal dust to build up inside the mine, where it was ignited by a spark from an improperly maintained piece of cutting equipment. Clogged and broken water sprayers then allowed what could have been just a flare-up to become an epic blast, the investigations found. Prosecutors said May manipulated the mine ventilation system during inspections to fool safety officials and disabled a methane monitor on a cutting machine a few months before the explosion. It wasn't clear from court papers whether the device was ever fixed. Although other mine accidents have led to criminal charges, prosecutors have typically targeted low-ranking employees. Quarles said he was surprised the charges reached so high into the ranks.