BECKLEY, W.Va. - They lived and they died pursuing the American Dream, working in dangerous conditions underground to help keep the lights on across the country, a somber President Barack Obama said Sunday in a eulogy to the workers who died in the worst mine accident in a generation.

The president told the families of the workers killed in the Upper Big Branch mine, about 35 miles from here, that the nation would honor their memories by improving safety in the mines.

"How can we fail them? How can a nation that relies on its miners not do everything in its power to protect them?" Obama said. "How can we let anyone in this country put their lives at risk by simply showing up to work, by simply pursuing the American Dream?"

With workers' families sitting near him - and the Massey Energy Co. executive who runs the mine sitting near the rear of the hall - Obama spoke broadly about the 29 workers killed in the explosion.

"In coveralls and hard-toe boots, a hardhat over their heads, they would sit quietly for their hourlong journey, 5 miles into a mountain, the only light the lamp on their caps, or the glow from the mantrip they rode in," Obama said.

"Most days, they would emerge from the dark mine, squinting at the light. Most days, they would emerge, sweaty, dirty, dusted with coal. Most days, they would come home. Most days, but not that day."

Investigators have detected high levels of two potentially explosive gases inside the mine, and it could be a month before investigators can get inside to determine what caused the April 5 blast. Federal regulators have identified highly explosive methane gas, coal dust or a mixture of the two as the likely cause of the blast, but the ignition source is unknown.

The explosion will be the subject of a Senate hearing on Tuesday, with the nation's top mine safety official expected to testify.

Obama has ordered a broad review of coal mines with poor safety records and urged federal officials to strengthen laws he previously called "so riddled with loopholes that they allow unsafe conditions to continue."