INDIANA (CBS 58) -- A 6-year-old Milwaukee girl was killed, and her mother seriously injured, following a crash in Indiana Wednesday, March 8.

Troopers responded to the crash on I-80/94 around 1:30 a.m. 

According to Indiana State Police, a semi truck pulling a trailer of U.S. mail rear-ended a vehicle occupied by two adults and three children, all visiting from Milwaukee. 

Police say all of the occupants sustained life-threatening injuries, including the driver of the Nissan -- identified as 27-year-old Ta'Quana Crawford of Milwaukee. All occupants were treated locally or flown out for specialized treatment. 

Authorities say one of the children, a 6-year-old girl, died while being transported. 

Indiana police say none of the three children were properly restrained in child safety seats. 

The driver of the semi -- identified as a 49-year-old man from Wheeling, Illinois -- was not injured. 

An investigation is ongoing.