GREYMOUTH, New Zealand — Hard rock layers slowed the progress of drills boring into a New Zealand mine as rescuers waited impatiently for a chance to test if air quality underground is safe enough for them to go in to pull out 29 trapped miners.

Family members have expressed frustration with the pace of the response as officials acknowledged for the first time it may be too late to save the miners, who have not been heard from since a massive explosion ripped through the Pike River Mine on the country's South Island on Friday.

A buildup of methane gas is the suspected cause of the explosion. And now the presence of that gas and others — some of them believed to be coming from a smoldering fire deep underground — are delaying a rescue over fears they could still explode.

A diamond-tipped drill was put to work on Tuesday as workers came within 33 feet (10 meters) of the tunnel where they believe some of the miners are trapped, police superintendent Gary Knowles said. The 500-foot (160-meter)-long shaft they are creating will allow them to sample gas levels — including explosive methane and carbon dioxide — and determine if rescuers can finally move in days after the blast.

Two workers stumbled out of the mine within hours of Friday's explosion, but there has been no contact at all with the missing 29. A phone line deep inside the mine has rung unanswered.

Officials will also feed a high-resolution laser camera down the hole to give rescuers their first sight of conditions — and potentially the men inside — said John Dow, the chairman of Pike River Coal Ltd., the mine owner. The camera will not be affected by the air quality.

Authorities are also hoping to send in a bomb-disposal robot into the mine. Army specialists worked through the night fitting the robot with a camera and up to 1.5 miles (2.5 kilometers) of fiber optic cable so it could take video of conditions in the tunnel.

The battery-operated robot can only operate in fresh air, and so cannot be sent into the mine until the air clears. Work was continuing to spark-proof the electronic robot so it would not cause a spark or anything else that could ignite flammable gases inside.

"Everybody's frustrated, everybody's upset," said Laurie Drew, whose 21-year-old son, Zen, is among the missing. "I have my moments I can keep it together but deep down my heart's bleeding like everybody else's."

Authorities appealed for patience.

"You can't put men underground as a rescue team until it's a safe environment," Knowles said Tuesday. "I've looked at other rescuers and the chance of rescue teams dying or being critically injured (in a fresh explosion) is great."

The youngest missing miner was so excited about his new job he persuaded mine bosses to let him start his first shift three days early — on the day of the deadly gas explosion — his mother told local media.

Joseph Dunbar was one day past his 17th birthday when he joined his fellow miners in the pit.

Mine shift supervisor Gary Campbell said Dunbar was desperate to be part of the team.

His mother, Philippa Timms, said her son "got offered this chance to have a career — and that's how he saw it, as a career," she told TV One.

The wait to begin the rescue bid for the men had been frustrating, but Timms said she understood why.

"They can't just rush in there because, I know right from the word go, I know how it works," she said. "If the oxygen rushes in and it hits that methane, then bam, they're gone, (in) another blast."

One of the two workers who escaped, Daniel Rockhouse, 24, described the explosion as being like an oversized shotgun blast.

He said the explosion smashed him into the mine wall and knocked him out. When he came to, he staggered to a nearby compressed air line to breathe in fresh air and gain some strength.

"I got up and there was thick white smoke everywhere — worse than a fire. I knew straight away that it was carbon monoxide," Rockhouse, whose brother Ben remains underground, was quoted as saying by the New Zealand Herald newspaper. "I couldn't see anything, and it was dead quiet. I yelled, 'Help, somebody help me!' But no one came. There was no one there."

Rockhouse stumbled toward the exit and eventually found the unconscious body of Russell Smith, the other survivor. Rockhouse began dragging Smith, until the other awoke. The two men stumbled through the dark haze to finally reach the surface nearly two hours after the explosion.

Police have said the miners, aged 17 to 62, are believed to be about 1.2 miles (two kilometers) down the tunnel.

Each miner carried 30 minutes of oxygen, and more fresh air was stored in the mine, along with food and water, that could allow several days of survival, officials say.

New Zealand's mines are generally safe. A total of 181 people have been killed in the country's mines in 114 years. The worst disaster was in March 1896, when 65 died in a gas explosion. Friday's explosion occurred in the same coal seam.

The Pike River coal mine differs from the Chilean gold and copper mine where 33 men were rescued after being trapped 69 days. Methane gas was not a concern at the Chilean mine, but its only access shaft was blocked, while the Pike River mine has two exits.