The 11th and 12th North entries had been driven toward a connection and stopped.  About noon on August 31st, a miner drilled through into the face of these entries and ignited gas in the hole with his open lamp.  An explosion occurred in the idle entries but did not affect the men who drilled the hole.

Smoke and gases were carried over men in 1st and 2nd North entries, resulting in the death of 11 men.  On September 2nd, an apparatus crew was exploring for fires during the investigation.  Three of the men became distressed, two being able to reach fresh air.  The other two carried the third man until exhausted.  When found 30 minutes later he was dead.

The spalling roof kept the road dust inert so that dust did not enter the explosion.  Source: USBM Annual Report Excerpt  (1.3 Mb)  

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Eleven Miners Lose Their Lives Because of Blackdamp
September 1, 1921

Harrisburg, Ill., Aug. 31. -- Twelve men of 434 miners in a mine of the Harrisburg Coal Company at Harco station, near here, are unaccounted for following a mine explosion this afternoon and it is believed they are entombed.

Shortly after 4 o'clock an official of the Harrisburg Company said he believed only seven or eight men were caught underground.  He said that of the approximately 450 men at work, all but a few were warned and got out in time.  He was unable to say in what manner the men believed caught are held.  He described the explosion as caused by gas.

Those miners rescued and who are expected to recover are William Sheely, William Cole, Ed Thomas, and four others whose names were not yet learned.

Volunteer rescue teams were organized by miners but their efforts were futile as blackdamp drove them back from the section of the mine where the men were entombed.

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Eleven Bodies Found
Waterloo Times-Tribune
September 1, 1921

Harrisburg, Ill., Aug. 31. -- Eleven dead miners' bodies were brought to the surface of the Harco mine from a depth of 445 feet, and 1,000 feet from the mouth of the mine at 6 o'clock tonight.  The entire number were entombed this afternoon when a dynamite charge in a new coal bed penetrated an old walled room, providing means of escape of blackdamp, which it is believed killed the miners.

A check of the miners at 5:30 o'clock this evening showed that 436 miners had entered the shaft this morning and that all but those dead had been returned to surface.  Warnings after the explosion by miners in a nearby shaft probably saved the lives of those who were sent to the surface.

The crew of 17 miners was engaged in working a new coal bed near an old shaft, which had been walled in, when the driller exploded a charge of dynamite which crushed the old vein walls, and entombed himself and fellow workers, according to the reports from the mine.

The bodies of the dead men are in local morgues tonight and await identification by their respective families.

A special train bearing a United States bureau car was dispatched to the mine within an hour after the accident and was in charge of D. J. Parker, chief of the division of mine rescue cars and stations of the U. S. Bureau of Mines, who was conducting an inspection trip in this vicinity for H. Foster Bain, director of the United States Bureau of Mines.

Official list of the deceased:

• David Stewart, miner, age 42, married, leaves a widow and six children
• Herbert Reeder, miner, age 28, married, leaves a widow and three children
• Herschel Vaughn, miner, age 26, married, leaves a widow and two children
• John Luther, miner, age 48, married, leaves a widow and one child
• Hiram Brown, miner, age 27, married, leaves a widow and two children
• Lyman Bulkley, miner, age 18, single
• George Hunter, miner, age 32, married, leaves a widow and one child
• George Warwick, miner, age 30, single
• Ernie Goodrich, miner, age 34, widower, leaves three children
• Mike Mosco, miner, age 38, married, leaves a widow and four children
• Charles Mosco, miner, age 36, single

Source document  

Seven men who escaped the explosion but are suffering from the effects of blackdamp were taken to local hospitals.  It is believed they will recover.  The mine will be sealed tomorrow by state authorities who will issue a permit after an investigation authorizing the continuance of operations.