Tests on air packs recovered from the depths of the Sago Mine show that none of the devices had been used to their full capacity before being discarded by the trapped miners, a federal mine safety official Thursday.

Though the devices, known as self-rescuers, activated when the 12 men tried them, the tests revealed that the amount of chemicals used to create oxygen varied widely, from just 25 percent to 75 percent, said John Urosek, a ventilation expert with the federal Mine Safety and Health Administration.

The revelation, during a third day of hearings into the Jan. 2 explosion, triggered angry questions from the son of one of the 11 miners who died during the 41 hours it took rescuers to reach them.

"None of them was used up 100 percent, and that should tell you that ... that's unacceptable," said Russell Bennett, the son of Marty Bennett and a coal miner.

It is the job of federal investigators to find out why they were not fully used, he said.

"And I agree with you," Urosek replied. "I think that's a question in all of our minds, as to why they wouldn't have been used fully, and to understand what the analysis actually means."

In a letter to the families of those killed in the mine, survivor Randal McCloy Jr. wrote that four of the air packs did not work and the 12 men trapped underground were forced to share. The air packs are designed to provide up to an hour's worth of oxygen.

Neither MSHA nor state investigators have interviewed McCloy, who Urosek said is the key to questions surrounding the air packs.

For now, investigators know the air packs "did react. They did start when the miners tried them," Urosek said.

At least two miners who were part of a second crew in the mine and escaped the blast told investigators they, too, struggled with their air packs.

West Virginia lawmakers approved legislation earlier this year that requires mining companies to store extra air packs underground. A similar proposal has been included in pending federal legislation, and MSHA is in the midst of receiving public comments on an emergency regulation that would require mine operators to have caches of air packs in all mines.

Neither MSHA nor the state Office of Miners' Health Safety and Training have completed their investigations. Gov. Joe Manchin has said he wants the state's report completed by July 1, but officials have said it may be delayed.

MSHA has ruled out several ignition sources for the blast, but not lightning or roof falls, said Richard Gates, the agency's investigation team leader.

An explosive concentration of methane gas "could easily be ignited by a weak electrical spark," as weak as the static electricity generated by walking across a carpet on a dry day, Gates said.

Mine owner International Coal Group has said it believes lightning hitting a tree more than a mile away from the mine's opening was able to travel into the mine and spark the blast in a section of the mine that had been abandoned and sealed in December.

The explosion occurred about 6:26 a.m. as a severe thunderstorm moved through the area. McCloy was the only survivor from the 13-member team closest to the blast; one miner died immediately after the explosion.