A bomb-disposal robot sent underground to learn the fate of 29 workers missing for four days in a New Zealand coal mine swirling with toxic gas short-circuited and failed Tuesday, sending hopes for their survival even lower.

Rescuers had another setback Tuesday when a team boring a narrow passage to an area deep underground where the miners might be struck hard rock that slowed the drilling.

Prime Minister John Key became the latest official to express the growing pessimism over whether the miners could have survived during the aftermath of the massive blast that ripped through the Pike River mine on Friday.

"We hope and pray that the missing men are alive and well," Key said in a solemn address to Parliament on Tuesday. "But given we have not had contact with the men for nearly four days, the situation remains grave. Although we must stay optimistic, police are now planning for the possible loss of life."

Army specialists deployed a remote-controlled robot into the mine overnight Monday. It was supposed to crawl more than 1 1/2 miles (2.5 kilometers) carrying sophisticated air quality testing equipment and a camera and give the first clues to the status of the men inside.

But water got into the robot sometime within the first two hours of its operation and it went out of commission, said police superintendent Gary Knowles, the head of the rescue operation. Replacement robots were being requested from West Australia and the United States.

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 — have prevented rescue teams from entering the mine because of fears they could still explode.

"I won't send people in to recover a robot if their lives are in danger," Knowles told reporters. "Toxicity is still too unstable to send rescue teams in."

A 500-foot (160-meter)-long shaft is being drilled toward an area where the miners may be. Once completed, the shaft will sample gas levels and determine if rescuers waiting impatiently can finally move in. Rescuers also want to drop a listening device down the hole to see if they can hear anything — such as tapping sounds — that might indicate that the miners are still alive.

A diamond-tipped drill came within 33 feet (10 meters) of the tunnel where some of the miners may be before progress was slowed by hard rock.

"This is a very serious situation and the longer it goes on, hopes fade, and we have to be realistic. We will not go underground until the environment is safe," Knowles said.

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

"The families are showing grief, frustration and probably anger," 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."

Those missing include a teenager who 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 explosion — his mother told local media.

Joseph Dunbar was one day past his 17th birthday and the youngest of the miners.

His mother, Philippa Timms, said the wait to begin the rescue bid had been frustrating, but that she understood why it was taking so long.

"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, another blast."

Police have said the miners, aged 17 to 62, are believed to be about 1 mile (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 them to survive for several days, 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.


Lilley reported from Wellington, New Zealand.