The Boone County sheriff's office confirmed multiple ambulances were called to the scene. Several people were seen being taken away on stretchers.
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The theater's marquee and facade crashed onto State Street, littering the area with bricks and debris as first responders scrambled to help the wounded.
WATCH | See inside Apollo Theatre in Belvidere after roof collapse
Video shows destruction inside Apollo Theatre in Belvidere after roof collapse
Around 260 people were inside the theater to see metal bands Morbid Angel, Revocation and Skeletal Remains. The show was scheduled to start at 7 p.m.
"We are currently sheltering in place, and want to extend our support and hope that everyone at the show tonight is safe," Morbid Angel wrote in a Facebook post. "Right now our focus is on making sure everyone in the venue tonight is ok and gets home."
WATCH | Metal fan waiting to hear from friends who were inside collapsed theater
Metal fan waiting to hear from friends who were inside collapsed theatre
Theresa Best, of Woodstock, was on her way to the concert when friends began calling her frantically to let her know what had happened. She said she has friends who were inside the theater that she hasn't been able to reach.
"The metal community is so nice, and my heart just aches for anybody who did lose their life," Best said. "I just hope all my friends are OK."
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Belvidere Fire Chief Shawn Schadle said first responders were on the scene within two minutes, since the fire station is just across the street.
Chief Schadle added that MABAS (Mutual Aid Box Alarm System) Rescue Team was performing a second, more thorough search and working to stabilize the building.
WATCH | Intense storm cell hits Belvidere, Illinois
Belvidere, IL theater roof collapses during intense storm
ABC7 Meteorologist Cheryl Scott said an intense storm cell hit the Belvidere area around 7:45 p.m., with winds in excess of 50 mph.
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Scott said it's possible a tornado may have touched down, but National Weather Service crews will have to survey the damage to confirm that.
Gov. JB Pritzker tweeted that his administration is closely monitoring the situation and is in touch with local officials.
This is a breaking news story. ABC7 Eyewitness News has a reporter and crew on the way to the scene. Check back for updates.