GREYMOUTH, New Zealand >> Hopes waned Monday for the survival of 29 New Zealand coal miners who have been trapped for three days underground, where the presence of explosive gases has prevented a rescue.

Family members 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, though officials say that may not be confirmed for days. 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.

“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 are working on drilling a 500-foot-long, six-inch-wide shaft into the mine tunnel to get a better idea of the air quality in areas where miners were believed trapped by the blast.

Officials will also feed a very 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.

Once the question of air quality is resolved, rescuers hope to send a bomb-disposal robot into the mine. Army specialists were at the mine site Monday fitting the robot with a camera and up to 1.5 miles 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. Also, checks were under way to make sure the robot would not cause a spark or anything else that could ignite flammable gases inside.

“We still remain optimistic, we’re still keeping an open mind,” police superintendent Gary Knowles told reporters. “But we are planning for all outcomes, and as part of this process we’re planning for the possible loss of life as a result of what’s occurred underground.”

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.

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.

“It wasn’t just a bang, finish, it just kept coming, kept coming, kept coming, so I crouched down as low as I could in the seat and tried to get behind this metal door, to stop getting pelted with all this debris,” Smith told TV3. “I remember struggling for breath. I thought at the time it was gas, but … it was dust, stone dust, I just couldn’t breathe. And that’s the last I remember,” he said.

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.

In China — which has the world’s deadliest mines — water flooded a small coal mine Sunday, trapping 29 workers. All of them were lifted to safety on Monday, state media reported.

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.