A tiny microphone lowered deep into the earth early Friday picked up no evidence that six coal miners are alive four days after they were caught in a cave-in.

However, crews drilling a hole for the microphone and other gear might have missed the chamber where the miners are believed to be trapped, and an air sample indicated the miners had enough oxygen to breathe if they survived the collapse, officials said around midday.

Using a steel drill bit to bore a 2 1/2-inch wide hole more than 1,800 feet into the mountain site of Monday's cave-in, rescuers reached their targeted spot late Thursday.

But the drill might have drifted into a neighboring sealed chamber.

"The advantage of the 2 1/2-inch hole is that it's fast. The disadvantage is that it's not as accurate," said Richard Stickler, head of the Mine Safety and Health Administration.