Aryanna Campbell was walking with friends Sunday evening when the arrow hit her from behind, said Mick Fink, Fond du Lac County Sheriff.
"It certainly appears to be a stray arrow, but at any minute that could change obviously," he said. "We are hoping the people in the area will come forward."
The arrow had a target tip on it rather than a more destructive hunting tip, Fink said, "which is a good thing otherwise it could have been a fatal impact."
Police canvassed the neighborhood and confiscated a bow and arrows from a man who lives in the apparent path of the arrow. Those items are undergoing analysis at the state crime lab to determine whether they are related to the shooting of Aryanna, Fink said.
The 7-year-old underwent surgery and is recovering in the hospital this week, her grandmother Michele Schneeberg told ABCNews.com.
"It was very scary. She was in real bad shape," she said of the moments after Aryanna was hit.
Schneeberg said the girl has taken a few steps and can't talk but is "still on pretty heavy pain medication."
She said she is just as baffled by the arrow strike as police.
"I think it was somebody doing something without thinking," she said. "Even if it was an accident, please own up to it."