TV footage showed emergency personnel carrying out injured concert-goers on stretchers, while video posted on social media showed waist-high rubble on the floor of the concert venue, and a gaping hole in the roof.
Belvidere Fire Chief Shawn Schadle reported one death and 28 injuries, including five people hospitalized with serious injuries.
In the neighboring state of Indiana, three people were killed by a storm in Sullivan County, on the border with Illinois, several US media reported, citing local authorities.
Overnight tornadoes also claimed one life in Pontotoc County, Mississippi, and one in Madison County, Alabama, emergency officials reported Saturday.
More than 600,000 homes were without power Saturday, according to the poweroutage.us website.
As the storm tracked north-eastward, the highest number of outages on Saturday afternoon were in the states of Ohio and Pennsylvania.
Several mid-Atlantic states were under high wind warnings.