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"One of the bears had a rock about the size of a basketball and managed to break one of the windows," Tony Fisher, director of animal collections for the Minnesota Zoo, told ABC News.
The three bears have temporarily been removed from the enclosure.
"At no time was there any danger of the bears getting out since there were still four layers of glass, but we're not going to trust it," Fisher said.
Witness Robin Ficker estimated the rock weighed about 50 pounds.
"The noise of the rock into the glass was so loud that it reverberated, and that is when people ran," Ficker told CNN. "He was using a rock as a tool ... He hit the window five times."
Zoo spokeswoman Kelly Lessard downplayed the incident.
"The brown bear was exhibiting typical curious natural behavior, interacting with a found object just like it might in the wild," she said.