HUNTINGTON, Utah (CNN) -- Efforts to reach six Utah coal miners trapped since Monday were taking longer than expected, a mining official said Thursday.
A 2½-inch drill hole was expected to reach the cavity where the miners are thought to be by midnight MT Thursday night (2 a.m. ET Friday). Earlier, Robert Murray, CEO of Murray Energy Corp., the parent company of the Crandall Canyon Mine, said a breakthrough would come Thursday evening.
The drill hole, made by a helicopter-borne drilling rig, had gotten through 1,730 feet of earth by 6 p.m. MT (8 p.m. ET). Officials said the drill would pierce through the remaining 140 feet early Friday.
"They know we're on the way," Murray said earlier. "Those men probably can hear that drill rig right now, if they're alive."
A second drill -- more than 8 inches in diameter -- began boring Wednesday morning. It was expected to reach the miners' cavity Friday evening, Murray said. Watch how trapped miners would communicate with rescuers »
Its start was delayed because rescuers first had to build 8,000 feet of road so the massive piece of equipment could be taken to the site. In addition, the rig had to be positioned on the mountain at a 23-degree angle. See where the miners are thought to be trapped »
Murray said the trapped miners probably would use a piece of metal to tap on bolts sunk into the mine's roof to let rescuers know where they are. Mine officials said they had audio and visual equipment on site to put into the hole to help find the miners.
Once the cavity is breached, "there is the possibility we may not learn anything conclusive," Rob Moore, vice president of Murray Energy, said Thursday.
"It may hit in an area that the miners are not able to get to. We're hopeful that we hit the void and the miners are able to go to the two-way communication devices and let us know their well-being," he said.
Murray cautioned again at Thursday's news conference that that the helicopter-borne drilling effort might not be successful.
Because there are no roads to the area above the mine, which is about 140 miles south of Salt Lake City, a helicopter guided by Global Positioning System data dropped the drill into location.
But he said the 2½-inch drill could bend at the depths it is now being used, and possibly miss the cavity where the miners are.
If that turns out to be the case, "we would have to start drilling again," he said.
Besides communication, the 2½-inch hole made by the rig placed atop the mine would enable authorities to get the miners air, water and food, buying time for an underground rescue.
Establishing an opening large enough to get the miners out will take six to seven days, Murray said Thursday.
While the mining company has not released the miners' names, family and friends have confirmed to CNN the identities of four of them as Kerry Allred, Carlos Payan, Brandon Phillips and Manuel Sanchez.
Relatives of Sanchez -- a coal miner for 17 years -- have complained about Murray's treatment of them since the collapse.
His sister, Maria Buenrostro, said it has been difficult for her family to get accurate information about the fate of her brother.
Buenrostro said that Murray stormed out of a meeting after family members started asking hard questions.
"We get upset, and he gets angry and he leaves," Buenrostro said. "That's wrong."
She said it was made more difficult because not everyone speaks English. Murray said he has taken steps to make sure information also is given to relatives in Spanish.
Murray has said that seismic activity from a magnitude-3.9 earthquake caused the cave-in. That opinion was not shared by geophysicists, who said the seismic activity they measured appeared to have been caused by the collapse of the mine itself.
Based on an analysis of data, the shaking that was detected bore the signature of a collapse and "not a tectonic earthquake," University of California-Berkeley seismologist Douglas Dreger said in a statement Thursday.
Experts have said the so-called "aftershocks" could be the rock adjusting after the collapse.
Murray denied reports that the men had been involved in "retreat mining," a dangerous practice in which pillars of coal holding up the ceiling of a mine are destroyed in an effort to dislodge more coal. "Retreat mining has absolutely nothing to do with the situation," he said. "This was introduced to the media by individuals who have no knowledge of what's going on." See the rescue efforts outside the mine »
About 50 representatives of the Mine Safety and Health Administration are on site, said Richard E. Stickler, assistant secretary of the Department of Labor for mine safety and health.
He said the mine is in compliance with federal laws.
Inspectors cited it for 30 violations this year, Mine Safety and Health Administration records show. Recommended fines in the 10 cases involving penalties ranged from $60 to $524.
In the past three years, the mine was cited at least 300 times -- with 118 of those citations for violations serious enough to cause death, records show. E-mail to a friend
CNN's Gary Tuchman, Ed Lavendera and Vivienne Foley contributed to this report.