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Six people were taken to hospital after the explosion blew the top off a shopping plaza in Waukegan, about 65km (40 miles) north of Chicago.
It was not clear what caused the explosion, which happened at around 1230 (1830 GMT) above a mobile phone shop and hair salon.
Waukegan Police Chief Bill Biang told the Chicago Tribune newspaper that the blast "appears to be a gas explosion".
TV footage showed a collapsed ceiling and shattered shop windows as emergency services rushed to the scene.
Fire crews from several towns were helping with rescue efforts, searching to see whether people are trapped inside the building.
A team from the People's Gas company were on the scene to investigate.
Holy snap LMZ! you are getting more dangerous!
It was not clear what caused the explosion, which happened at around 1230 (1830 GMT) above a mobile phone shop and hair salon.
Waukegan Police Chief Bill Biang told the Chicago Tribune newspaper that the blast "appears to be a gas explosion".
TV footage showed a collapsed ceiling and shattered shop windows as emergency services rushed to the scene.
Fire crews from several towns were helping with rescue efforts, searching to see whether people are trapped inside the building.
A team from the People's Gas company were on the scene to investigate.
Holy snap LMZ! you are getting more dangerous!