"It's a problem because there's a lot of phones at a mine, like the guard shack and various mine offices," Sparks said. "You can get to different phones that you try to monitor, but before you get to the other ones, they can call in ahead of you."
Both of the Kentucky cases were part of a 57-mine inspection blitz launched in the days following the April 5 Upper Big Branch disaster. The agency has targeted mines with ventilation problems, high methane levels and buildup of coal dust -- factors believed to have triggered the massive explosion at Upper Big Branch.
That theory was bolstered on Thursday when MSHA said a handheld meter found deep inside the Upper Big Branch detected explosive levels of methane before the blast.
Carol Raulston, a spokeswoman at the National Mining Association, said MSHA's response has been overly aggressive considering that most mines have a safe track record.
"MSHA's high public profile on this inspection technique is offensive to the vast majority of U.S. mines that are trying their best to comply with all safety requirements and to improve miner safety," Raulston said.
"The conditions we're finding when we're able to circumvent some of these intended advance notices are just appalling," Main said.
In some cases, ventilation curtains had been removed, miners had not removed dangerous piles of rock dust or workers were mining in areas where they were not permitted, Main said.
Current law provides for up to a $1,000 fine and imprisonment up to six months for anyone giving advance notice of an inspection. A mine safety bill working its way through the House would boost the prison term up to five years and raise the fine up to $250,000 for individuals and $500,000 for corporations that knowingly give advance notice to impede an investigation.
In the meantime, MSHA is working to "change the culture in the mining industry," Main said. "Showing up when we're least expected is a tool that's been used and will continue to be used."
AP writer Tim Huber in Charleston, W.Va., contributed to this report.