Authorities say a Fond du Lac duplex fire that happened Monday night was caused by arson.Shortly after 11:00 p.m. on Monday, first responders were called to the 200 block of Packer Street in Fond du Lac to find a duplex on fire. Crews eventually put the fire out, but authorities say the blaze caused extensive damage to the building.During the fire, officers located a 16-year-old girl trying to escape from the second floor of the duplex. She climbed out of a window but was hurt in the process. The teen is now recovering at Children's Hospital in Milwaukee and is in stable condition.Investigators found that an 'improvised incendiary device' was used to target the home. Authorities determined the fire was caused by an act of arson.Law enforcement arrested a 60-year-old man in connection to the fire on Wednesday. The suspect is currently being held at the Fond du Lac County Jail.Authorities did not release the names of the injured teenage girl or the suspect.The Wisconsin Department of Justice Department of Criminal Investigations assisted Fond du Lac's police and fire departments.Stay with NBC 26 on-air and online as we continue following this story.