A woman with the group and three Girl Scouts - including her daughter - were killed, and a fourth girl was critically injured.
A doctor who examined the youngster said his injuries had been deliberately inflicted, with Chippewa County Sheriff's Office quickly focusing its investigation on the 10 year-old girl.
"She stated she was holding the child and that's when she dropped the child", Wade said.
The 10-year-old then "didn't know what to do and didn't want to get into trouble and then she proceeded to stomp on the 6-month-old's head", he said.
Treu appeared in court via closed-circuit television Monday.
The 11 charges filed against Colten Treu on Tuesday 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?"
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Judge James Isaacson ordered a $50,000 cash bond, calling it "an appropriate resolution", while adding that a lot would happen in the ensuing 48 hours in determining where the girl will be placed.
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.
The girls were students at the Halmstad and Southview elementary schools in the Chippewa Falls School District and members of Troop 3055.
Police questioned one adult and three children who were present at the day care center. The truck was found to be abandoned in the garage at this location and the driver and a passenger had left the house prior to police arriving.
CNN has been unable to reach Treu, who does not yet have an attorney. The other girl was initially reported to be in critical condition.
Police say they were hit over the weekend when the pickup veered off a rural highway in Lake Hallie as they collected trash along a rural highway and then sped away.