Elation over a possible rescue attempt quickly turned to anguish for the families of 29 New Zealand miners missing underground since last week when a second powerful blast ended any hope for another mine miracle.

Wednesday's massive explosion deep inside the mine on New Zealand's South Island came five days after the men were caught underground by a similar blast. Even in the unlikely event that any had survived the first one, police said none could have lived through the second.

"The blast was prolific," said police superintendent Gary Knowles, in charge of the rescue operation, "just as severe as the first blast."

Still, as families gathered for their daily briefing on the rescue operation's progress, they hoped for the best. Before he could finish telling them about the latest blast, some broke into applause when Pike River Coal Chief Executive Peter Whittall said a team had been getting ready to go underground — thinking that a rescue was about to start.

"I had to wait till they stopped clapping to tell them ... that the second explosion occurred," Whittall said.

Some relatives collapsed. Others shouted at the police in anger.

"It is our darkest day," said Tony Kokshoorn, the mayor of the nearby town of Greymouth, who was at the meeting.

Prime Minister John Key declared the disaster a national tragedy.

Both blasts were believed to have been caused by explosive, toxic gases swirling in the tunnels dug up to 1½ miles (two kilometers) into a mountain that had also prevented rescuers from entering the mine to search for the missing. Officials said the second blast could not have been prevented and was not a result of any of the rescue activities.

It was one of New Zealand's worst mining disasters. The country's industry is relatively small compared to other nations and considered generally safe, with 210 deaths in 114 years after the most recent tragedy.

It devastated families who — buoyed by the survival tale of Chile's 33 buried miners — had clung to hope for more than five days that their relatives could emerge alive.

Key said flags would fly at half staff on Thursday and Parliament would adjourn its session in respect for the dead men.

The second blast came hours after the first progress in days for the rescue attempt, when a drilling team broke a narrow shaft through to the section of the mine where the missing workers were believed to have been. And two robots crawled their way into the tunnel, giving authorities their first view of the inside of the mine.

But officials had become increasingly pessimistic about the chances of pulling the men alive from the mine. Nothing had been heard from them since the initial blast.

Officials said investigations still to come would confirm the exact cause of Wednesday's explosion.

Whittall said rescue teams were not doing anything that could have set it off, and conditions inside the mine were such it could have happened at any time.

"It was a natural eventuation," he said, "it could have happened on the second day, it could have happened on the third day."

Laurie Drew, father of 21-year-old miner Zen, said rescuers should have gone into the mine on Friday. He believes the first explosion would have burned off most of the dangerous gases.

"They had their window of opportunity that Friday night, and now the truth can't come out because no one alive will be able to come out and tell the truth about what went on down there," Drew said. "The only thing that's going to make matters worse is if we find ... out that people were alive after that first blast."

Knowles said at all times after the initial blast, entering the mine was simply too risky because of high gas levels and evidence of a smoldering coal fire underground that could have been the ignition source.

Energy Minister Gerry Brownlee said a range of official inquiries would probe the cause of the disaster and whether it could have been prevented.

Whittall said no decision had been made yet on whether the mine would be sealed or what the next step would be.

"We can't still go into an unsafe mine. It's just as unsafe now as it was two hours ago, the gas will still be coming out of the coal, there's still an ignition source, there's no doubt burning methane from that explosion," he told reporters. "But we want our boys back and we want to get them out."

New Zealand's mines have been safe historically. The worst disaster was in 1896, when 65 died in a gas explosion at a mine on the same coal seam as the latest tragedy. The most recent was in 1967, when an explosion killed 19 miners in a mine near the Pike River site.

While conditions were different, the New Zealand disaster drew initial comparisons with the rescue last month of 33 workers trapped 69 days in a gold and copper mine in Chile.