BECKLEY, W.Va. -- Federal investigators looking into the deadly blast at the Upper Big Branch mine know that someone bypassed a methane monitor on an underground machine weeks before the explosion, but are now faced with two very different accounts of what happened afterward.

Massey Energy, the mine's owner, said the monitor on the machine was bridged, or bypassed, using an electrical conductor, to allow workers to move it from an active section so the monitor could be repaired.

Other witnesses have said the machine continued to mine coal -- something that, if true, would directly violate federal laws governing mine safety.

The monitors are hooked into the control panel of the machines, called continuous miners, which claw coal from the seams.

Because coal seams often emit explosive methane gas, the monitors are designed to shut off the machine when methane levels rise.

In a statement last week, Massey said miners disabled the monitor only to move the machine to a safer place for repairs. But mine safety experts called that explanation questionable at best, saying it is never legal to bridge a methane monitor.

Federal investigators, who are conducting a criminal probe of the April 5 blast that killed 29 miners, were told by former Upper Big Branch employee Ricky Lee Campbell and other witnesses that electrician George Holtzapfel was ordered to loop a circuit around a methane detector on a continuous mining machine.

Last week, Jonathan Price, an attorney for Mr. Campbell, told the Pittsburgh Post-Gazette that his client saw that occur and has received a subpoena to testify before the federal grand jury investigating the blast.

"My client has said that they continued to operate the continuous miner after bridging the detector," Mr. Price said.

Mr. Holtzapfel last week confirmed to the Post-Gazette that he had bridged the monitor after protesting a supervisor's order to do so.

"I've worked for probably six or eight coal companies. They're all the same. They all do the same practices. It's not something that was new, it was just new to me," Mr. Holtzapfel said.

In its e-mailed statement, a Massey Energy spokesman confirmed the Feb. 13 incident ---- which occurred in the mine four miles from the section that blew up April 5 -- but said it was done only to move the machine to a safer area for repairs.

"The methane monitor was bypassed in order to move the [continuous] miner from the area that did not have roof support to a safer area for repair," Jeff Gillenwater said in the statement.

Newer continuous miners, Mr. Gillenwater noted, are fitted with a manual override system that can be activated in the case of a malfunction to move the machine to another area for repairs.

Some witnesses have asserted to federal investigators that the company puts coal production ahead of ensuring safe conditions for workers. Mr. Gillenwater, in the statement, said Massey "strongly forbids any improper conduct" regarding safety devices.

"If additional claims of wrongdoing are raised during this investigation, Massey will fully investigate those matters and take appropriate action," he said.

The company's account, however, is at odds with Mr. Campbell's account to investigators that the monitor was bridged in the morning and the mining machine operated there for the rest of the day.

Attorney William Cline of Charleston, W.Va., is representing the family of one of the miners who died in the explosion -- the nation's worst mine disaster in 40 years. He said Massey deserves the benefit of the doubt at this stage of the investigation, but he isn't ready to accept the company's explanation.

"I look at all of that with a little bit of a jaundiced eye to some extent," Mr. Cline said. "That's a pretty convenient thing to say."

Mine safety attorney Tony Oppegard, a former federal Mine Safety and Health Administration official, was more skeptical.

"The bottom line is that if your methane monitor isn't working, you repair it," he said. "You don't bridge it out. You repair it."

Though the practice is illegal, bridging detectors is not uncommon underground, according to Mr. Oppegard and Upper Big Branch workers.

"It's done frequently because a lot of companies don't want to slow down production," Mr. Oppegard said. "They will bridge it out until the end of a shift and get someone to fix it. It's easy to do and hard to catch someone doing it because you can remove the bridge in 30 seconds."

In Washington, for a hearing on mine safety legislation, former Upper Big Branch miner Stanley "Goose" Stewart told the Post-Gazette that he heard bridging was a pervasive practice at the mine though he never personally witnessed an incident.

He testified at the hearing that at least two fireballs had been spotted in the mine before April 5.

"How could methane build to that point where a fireball could start?" Mr. Stewart said. "How could this happen if methane detectors had been working?"

Upper Big Branch investigators are looking into incidents of bridging beyond Feb. 13 to examine an apparent culture of cutting corners on safety in the mine.

Washington correspondent Daniel Malloy writes the "Pittsburgh On The Potomac" blog exclusively at PG+, a members-only web site of the Pittsburgh Post-Gazette. Our introduction to PG+ gives you all the details.