According to USAC, the 30-year-old driver was airlifted to Indianapolis' Methodist Hospital. He's listed in critical but stable condition.
Hmiel's No. 17 car reportedly slammed into the outside wall between turns three and four and then cart-wheeled violently. Hmiel sustained head, neck and back injuries as the car's protective roll cage was damaged as it hit the wall.
"We are very concerned about Shane and offer our resources and our support to his family during this time of healing," USAC President/CEO Kevin Miller said in a statement.
Hmiel was banned from NASCAR in 2006 following his third failed drug test. He competed in the Nationwide Series and Camping World Truck Series from 2002-2005 prior to his ban. Hmiel's lone victory came in the Truck Series' Las Vegas 350 in 2004.
Rehabilitated and sober, Hmiel began competing in all three of USAC's national touring series: the USAC Silver Crown Series, the USAC National Sprint Car Series and the USAC National Midget Series. He won USAC's "Most Improved Driver" of 2009.
Hmiel's father, Steve, is a competition director for the Earnhardt-Ganassi Racing team.
Information from the Associated Press was used in this report.