The baby was airlifted to a hospital, and "specialists indicated the injuries were consistent with child abuse, not by accident circumstances", the release states.
Deputies say the 10-year-old confessed to her involvement in a November 2 interview.
The girl was in foster care at a home in Tilden, a town almost 200 miles northeast of Madison that also serves as a day care centre, when she accidentally dropped the infant who hit his head on a stool and began to cry, WEAU News reported, citing Chippewa County District Attorney Wade Newell. 6, 2018, 11 charges were filed against Colten Treu include four counts of vehicular homicide and four counts of a hit-and-run resulting in death.
Judy Schneider, the mother of Sara Jo Schneider and Haylee's grandmother, told The Associated Press in an interview Monday that after the crash she wondered, "What was the driver thinking?"
Colten Treu, 21, sped off after the collision in Lake Hallie Saturday morning, but later surrendered.
Lake Hallie Police said Mr Treu and a passenger in the pickup truck both told investigators they had intentionally been inhaling chemical vapours just prior to the crash.
Lake Hallie police Sgt. Daniel Sokup said Saturday that Treu's black Ford F-150 crossed a lane Saturday morning and veered into a roadside ditch, striking the victims. A judge set her bond at $50,000.
The fourth-graders from the Chippewa Falls School District were picking up trash along County Highway P as part of a community service project when they were struck.
Police questioned one adult and three children who were present at the day care center.
Haylee also enjoyed painting and working on clay pottery with her mother.
According to Kowalczyk, the girl said she was holding the baby when she dropped the infant, who hit his head on a footstool. He says Treu does not yet have an attorney.
Sherri Jasper, a Girl Scout board member and counselor at Halmstad Elementary School, leads the program for a candlelight vigil Sunday at the school in Chippewa Falls, Wisconsin.