GREYMOUTH, New Zealand — Relatives of 29 miners still missing three days after an underground explosion were given a tour of the site Sunday in order to better understand the situation, including how toxic gases are preventing a rescue attempt.

Nothing has been heard from the men who were deep in the mine near Atarau on South Island when a massive blast ripped through it late Friday, but officials insist it is still a rescue operation.

“We remain focused on these guys and bringing them home,” rescue controller, police superintendent Gary Knowles, told a news conference Sunday in the nearby town of Greymouth.

But with levels of dangerous gases still fluctuating within the mine tunnel, it was too dangerous to send in the rescue teams that have been on standby since the explosion.

A buildup of poisonous gasses, which could be coming from a coal fire smoldering deep underground, could generate another explosion, officials said.

“I can assure you that every mines rescue man working on this operation is waiting to go underground to rescue our brothers. If there’s the slightest opportunity to go underground, we will,” said Trevor Watts, general manager of New Zealand Mines Rescue.

Anguished relatives have been desperately waiting for news or for rescuers to enter the mine to find their loved ones. They were taken on a two-hour tour of the site Sunday morning.

“It was obviously quite an emotional thing for them to go up there, where their family members are still undergound,” said Peter Whittal, chief executive of Pike River Mine Ltd. “They were very appreciative of that and have a lot better understanding of what we’re talking about.”

He said his talks with two men who escaped from the mine Friday had revealed no fresh information. Both were only part of the way into the mine when the explosion occurred.

“None of them were up far enough into the mine to know where the explosion initiated. All we know is ... it came further into the mine than where they were and that both of them got out,” he said.

Mine experts on Sunday began drilling a six inch-wide (15 centimeters) hole from the mountain above to test for levels of deadly gasses in the center of the mine. It could take up to 24 hours to complete the 500 foot (150 meter) hole into the mine.

“We’ve got a heating of some sort underground and that means there’s some combustion generating the gases that go with that, carbon monoxide, a slight increase in methane and some other gases,” Whittal said. “Something is happening underground, but what it is we don’t know.”

Fresh air is being pumped down an open air line into the mine.

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

Electricity in the mine went out shortly before the 3:45 p.m. explosion and that failure may have caused ventilation problems and contributed to a buildup of gas, officials have said.

Whittall said the blast was most likely caused by coal gas igniting.

Two dazed and slightly injured miners stumbled to the surface hours after the blast shot up the mine’s 354-foot (108-meter)-long ventilation shaft. They have declined to speak to media.

Comparisons are being drawn to the Chilean mine accident, where 33 men were rescued from a gold and copper mine after being trapped a half mile (one kilometer) underground for 69 days.

But mine safety expert David Feickert said unlike the Chilean mine, Pike River officials have to worry about the presence of methane. He added, however, that the Pike River mine has two exits, while the mine in Chile had only one access shaft that was blocked.

The coal seam at the mine is reached through a 1.4-mile (2.3-kilometer) horizontal tunnel into the mountain. The seam lies about 650 feet (200 meters) beneath the surface. The vertical ventilation shaft rises 354 feet (108 meters) from the tunnel to the surface.

Whittall said the horizontal tunnel would make any rescue easier than a steep-angled shaft, once safety is established.

Each miner carried 30 minutes of oxygen, enough to reach oxygen stores in the mine that would allow them to survive for several days.

Australian and British citizens were among the missing men, and Australia sent a team of mine rescue experts to assist the operation.

The 2-year-old Pike River mine is working the largest-known deposit of hard coking coal in New Zealand, about 58.5 million tons.

New Zealand has a generally safe mining sector, with 181 people killed 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.

Greymouth mayor Tony Kokshoorn said that the families were optimistic.

That’s just the way you are when ... you live in a small community,” he told TV3. “They are out there with only one vision — that the miners, are coming back to us. And until someone tells us otherwise, that’s the vision we’re going to cling hold of.”

————

Lilley reported from Wellington, New Zealand.