Police in Wisconsin said they fatally shot a gunman who opened fire inside a casino in Green Bay, killing two and injuring a third in what has been described as a targeted shooting.

The shooting broke out at the Oneida Casino and prompted a massive law enforcement response. 

Brown County Sheriff's Lt. Kevin Pawlak said investigators believe the gunman was targeting a specific person he was angry at, but the person wasn't at the casino at the time.

The gunman "decided to still shoot some of the victim's co-workers or friends, it appears," Pawlak said.

A witness told Fox 11 that he arrived at the casino's parking lot and spotted people running from the location. The witness – who described the sound of rapid gunfire – said they saw about 50 to 60 police cars rushing to the scene. 

The casino's crowd was likely larger than normal since Saturday was the running of the Kentucky Derby.

Videos posted on social media showed people fleeing into the parking lot, where multiple police vehicles can be seen with their lights flashing.

Max Westphal said he was standing outside after being evacuated for what he thought was a minor issue.

"All of a sudden we hear a massive flurry of gunshots – 20 to 30 gunshots for sure," Westphal told WBAY-TV

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Oneida Nation said it was shutting down all of its casinos until further notice.

The Associated Press contributed to this report