New Zealand held a national day of mourning Thursday for the 29 victims of its worst mining disaster in decades, while officials said it could be weeks or months before their bodies are recovered from deep underground.

Recovery teams were prevented from entering the mine by high levels of potentially explosive methane and other toxic gases that are blamed for two huge blasts inside the Pike River Coal mine.

The first explosion last Friday left the 29 men missing up to 1 1/2 miles (2 kilometers) underground, and officials insisted there was hope for them. After Wednesday's second blast, police and mine officials said no one could have survived.

New Zealand's mining industry is small and generally considered safe. The tragedy deeply shocked the country and devastated families who — buoyed by the survival tale of Chile's 33 buried miners — had clung to hope that their relatives could emerge alive.

Mine operators said Thursday they were determined to bring out the men's bodies.

"I still want them back and their families want them back and we'll be doing everything we can to make that happen. My love and support are with those guys," said Peter Whittall, the CEO of Pike River Coal.

But testing showed gas levels surged again soon after Wednesday's explosion and that another explosion was possible. Methane is leaking from the coal seam, and a smoldering fire somewhere in the mine is producing other toxic gas and a potential ignition source, officials say.

Mining experts were planning on expelling oxygen from the mine that could fuel a further explosion. One option was to pump inert gas into the mine to push the oxygen out, another was to seal the mine up to stifle any burning then enter when it was safe, Whittall said.

The recovery effort could take weeks, he said.

Prime Minister John Key said previous experience overseas suggests the operation could take months.

Key met again with relatives of the victims on Thursday, and promised them a thorough investigation.

"They have accepted that their loved ones are gone — but they want answers," Key told reporters.

Flags flew at half-staff across New Zealand, and special church services were held for people wanting to show respect for the miners. In Parliament, lawmakers wearing black passed a condolence motion.

It emerged Thursday that a rescue team was readying to go into the mine on Wednesday for the first time shortly before the second blast. A team was in full gear and ready to begin searching when gas levels suddenly spiked and the mission was called off. The explosion followed soon after, with no warning.

"She was all go" for the rescue effort, said Geoff Valli, whose brother Keith was among those killed. "They explained just how close they were to going in. It was bloody scary. It could have been so much worse," he told National Radio.

Laurie Drew, whose son Zen died in the mine, said the families wanted their loved ones' remains to help grieve.

"Hopefully it doesn't drag on too long to get the closure that all the families really need," he said.

A series of inquiries, including a formal Commission of Inquiry and police and coroner's investigations, are being launched into the mining disaster, one of New Zealand's worst.

The country has had 210 deaths in 114 years in mines. New Zealand's worst mine 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. A fire in a mine in 1914 killed 43.

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Lilley reported from Wellington, New Zealand.