Champaign County prosecutors believe Coffey, Thompson and two other men were shooting off a potato gun at Coffey's Cable Road home when the device exploded, killing Thompson.
Authorities said a potato gun is a pipe filled with accelerant that hurls potatoes or other objects when it is lit.
Coffey's guilty plea is part of a bill of information meaning he waived his right to have the case heard before a grand jury. When Coffey is sentenced, he faces between six and 12 months behind bars.
However, Champaign County prosecutors are recommending community control instead of jail time. Coffey will be sentenced next month.
Previous Story: June 27, 2004: Potato Gun Explodes, Kills One Man