Officials say he died from non-combat injuries.
He was part of the same National Guard unit based in Woodstock that lost three members in March during a roadside bomb attack.
Saczek was 23 years old, married and the father of a child.
The Guard said Monday that Saczek's death Sunday is under investigation, but didn't give additional details.
Saczek ws assigned to Company D, 1st Battalion, 178th Infantry based in Woodstock. Officials say he enlisted in the Illinois Army National Guard in July of 2006. He was on his first deployment.
The Illinois National Guard has had 28 casualties in Afghanistan and Iraq since operations began.
Officials say funeral arrangements haven't been made yet.