A shocking, gruesome accident on southbound Interstate 55 claimed two lives and seriously injured a dozen motorists Tuesday night.The accident happened around 6:15 p.m. near mile marker 27, when a semi plowed through seven other vehicles, all traveling at highway speeds.“This is one of the worst accidents I’ve seen my entire life,” said Geoffrey Schlechte, who was a member of the response team on scene. “This is terrible.”20-year-old Hailey Bertels was one of the two killed in the crash, along with her 17-year-old sister Madisen Bertels. Both were from Staunton.The eight-vehicle accident shut down the highway into the night as officials worked to uncover the cause of the crash. Stretches of the highway are under construction around the area of the accident, but the site of the crash was in a pocket without any work going on. “Where the actual crash happened there was a gap in between the construction,” said Illinois State Trooper Calvin Dye Hr. “We’re still investigating as to why the driver was unable to stop and why he struck those other seven vehicles.” At least two of the 12 injuries are considered life-threatening injuries.