Hours after an underground explosion, deadly carbon monoxide gas poured from the entrance to the Sago Mine at more than five times the level considered safe for humans, according to government documents released Friday.

The lethal gases were recorded by air-quality monitors a full two miles from the Jan. 2 blast that led to the deaths of 12 miners.

Documents posted Friday on the Web site of the federal Mine Safety and Health Administration offered more details about why rescue teams were prevented from entering the mine immediately after the accident.

Six hours after the explosion, the air monitors measured carbon monoxide, a byproduct of combustion, at 2,200 parts per million, more than five times the level considered safe for humans. The level peaked at 2,600 parts per million, before starting to drop, the documents said.

The first rescue crew entered the mine nearly 12 hours after the blast, but it took two days before the miners were found deep inside the mine about 100 miles northeast of Charleston.

The cause of the explosion has not been determined, but officials with the mine's owner, International Coal Group Inc., say it occurred about two miles inside the mine in an abandoned section that was sealed off last month.

ICG President Ben Hatfield said earlier this week that the Sago Mine was a "safe operation," despite 208 safety violations last year. Hatfield said none of the violations appeared to be related to the accident.

Also Friday, federal mine regulators released two safety violations issued to the mine in August following roof collapses that seriously injured one miner.

Ventilation of the mine continued Friday with an eye toward sending investigators underground within a week.

Meanwhile, the sole survivor of the disaster remained in critical condition at a Morgantown hospital. Randal McCloy Jr., who is believed to have suffered some brain damage from carbon monoxide poisoning, remained in a coma.

A public memorial service for the 12 miners who died was scheduled for Sunday in Buckhannon.