MULLENS, W.Va.  - As grieving relatives began burying some of the 25 coal miners killed in a massive underground explosion, crews prepared to go back into the mine Friday despite increasingly slim odds of finding survivors.

Rescuers pulled seven bodies from the mine just after Monday's blast, the worst U.S. mining disaster in two decades, but were forced out by poisonous gas before they could remove the rest or check for four missing miners who might have been able to hole up in refuge chambers.

Rescue teams have been trying ever since to get back inside Massey Energy Co.'s Upper Big Branch mine, but had to turn back for a third time Friday when they encountered smoke about 1,000 feet below the surface and five miles in. "We are praying for a miracle," President Barack Obama said as he offered his condolences to the victims' families in Washington on Friday.

Kevin Stricklin of the Mine Safety and Health Administration said at a briefing Friday afternoon that 16 rescuers would go back in Friday afternoon for a fourth try and hoped to get near the refuge chamber within three or four hours. Crews drilled a hole and had hoped to drop a camera in to check if the refuge chamber had been used, but they later determined that would not work.

Officials have not said what caused the blast, but they believe high levels of methane gas may have played a role. They also were not sure what was causing the smoke but said they pumped enough nitrogen into the mine to make it safe for crews to try again in the afternoon.

As crews prepared to go back in, more than 300 people packed the Mullens Pentacostal Holiness Church for the funeral of Benny Willingham, a 61-year-old miner who was five weeks from retiring when he died. He was remembered as a devout and generous man who recently gave a used car to a stranger. He had been a miner for more than 30 years and became a Christian 19 years ago this week.

"He wasn't just a weekend warrior," said the Rev. Gary Pollard of the Mullens Family Worship Center. In the days since the explosion, details have emerged about an extensive list of safety violations at the mine. Massey Energy has been repeatedly cited and fined for problems with the system that vents methane and for allowing combustible dust to build up.

CEO Don Blankenship has strongly defended the company's record and disputed accusations from miners that he puts coal profits ahead of safety. Federal regulators issued evacuation orders for all or parts of the mine more than 60 times since the start of 2009, according to a report prepared for Democratic Sen. Robert Byrd of West Virginia.

In 2007, the mine met criteria to be declared by the Mine Safety and Health Administration to have a pattern of violations. This would have allowed for stricter oversight by the federal agency, including the potential shutdown of the mine, but Massey was able to reduce the number of the most serious violations and avoid the declaration.

MSHA has appointed a team of investigators to look into the explosion, and Obama said he has asked federal mine safety officials to report next week on what may have caused the blast. "It's clear that more needs to be done," Obama said of mine safety. The U.S. House and Senate plan to hold hearings, though they won't set a date till rescue efforts are over. Byrd said lawmakers will scrutinize Massey's practices.

There have been no signs of life inside the mine since the day of the explosion, but officials and miners' families prayed the four miners somehow made it to a refuge chamber stocked with four days' worth of oxygen, food and water for more than a dozen miners. It's possible that with fewer miners inside, they could survive for longer than four days. Rescuers got far enough Friday to see that no one was in one of two chambers that had not been checked.

But as they tried to get to the last chamber in the morning, they found signs of fire and smoke and had to retreat before they could determine if any miners were inside. The refuge chamber is an expandable box activated by opening a door and pulling a lever. It takes about five minutes for the chamber to deploy, and crews using a camera or checking person should be able to see easily whether that happened, said Rory Paton-Ash, a spokesman for the manufacturer, Strata Safety. "You would know very, very clearly if it had been deployed," he said.

Search teams had gotten frustratingly close a day earlier to answers for the families of the missing miners - just 500 feet from the emergency chambers where any survivors would be - then were ordered to retreat because of volatile gas. Of the 25 confirmed dead, 18 bodies remain inside. Seven bodies were removed earlier in the week.

Two miners survived, and one of them remains hospitalized. Jennifer Renner, 22, of Charleston knew Cory Davis, who was among those killed in the blast.

Related -- Smoke, fear of fire push rescuers from W. Va. mine