WASHINGTON — Some mine companies are tipping off their underground workers before federal officials make surprise inspections, an illegal practice that has become more prevalent since a West Virginia explosion killed 29 miners, the nation’s top mine official said Thursday.

“We’re looking at this as a chronic problem without question,” Mine Safety and Health Administration director Joe Main told The Associated Press. “We have found enough evidence to know that we need to act to beef-up enforcement of the law to prevent this advance notice.”

Main’s comments came as his agency issued a special guidance bulletin to mines around the country clarifying the ban on giving advance notice of inspections.

The government has stepped up surprise inspections nationwide in the wake of the April explosion at Massey Energy’s Upper Big Branch mine in West Virginia. Some workers at the mine testified that managers found ways to tip off miners ahead of time so they could pass inspections. Massey officials have denied issuing any illegal warnings, but the company faces civil and criminal investigations.

Advance notice could give miners anywhere from 10 minutes to more than an hour to hide safety problems such as improper ventilation or disabled methane monitors while inspectors make their way from the main office to locations thousands of feet underground.

MSHA has already issued 28 citations for advance notice violations this year. It issued 31 for all of last year — the highest number in a decade.

To combat the problem, MSHA has turned to more aggressive tactics like commandeering the phones as soon as inspectors arrive or driving up in cars the mine company won’t immediately recognize. But it’s become a dangerous cat-and-mouse game as some mines post lookouts or install infrared beams that alert them when anyone enters the property.

“At some of these mines, there’s just one long dirt road where they can see you coming,” said Eddie Sparks, MSHA’s acting assistant district manager for enforcement in Barbourville, Ky. “Some of the coal truck drivers can get on the radio and call ahead before you ever get to the mine.”

Sparks said that’s what happened on April 19 when inspectors drove up to Manalapan Mining Co.’s RB No. 12 mine in Harlan County, Ky. Inspectors monitoring CB radio heard truck drivers alerting the company.

At another inspection the same day, MSHA officials seized control of phone lines as soon as they arrived at Left Fork Mining Co.’s Straight Creek No. 1 mine in Bell County, Ky. But Sparks said inspectors still overheard a mine employee on another phone calling down to workers to shut the belts off because inspectors were outside.

“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. The meter detected 5 percent methane in the mine’s atmosphere, according to Kevin Stricklin, MSHA’s chief of coal mine safety.

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.”

— Associated Press writer Tim Huber in Charleston, W.Va., contributed to this report.