Rakayo Alandis Vinson, 24, has been arrested in connection to the shooting and is currently being held at the Kenosha County jail, the sheriff's department announced Monday.
Cedrick Gaston, 26, Atkeem Stevenson, 26, and Kevin Donaldson, 27, were identified as the three men killed, according to the sheriff. At least three others were also wounded by gunfire.
WATCH | Kenosha County Sheriff provides update on Somers House Tavern mass shooting, arrest
Vinson is accused of opening fire at the Somers House Tavern off Sheridan Road in Wisconsin early Sunday morning.
Three of the victims were hospitalized with gunshot wounds, and police said there could be at least one other possible unknown shooting victim.
Two of the victims died at the scene. The third victim fled in a car with other people but later died on the way to a hospital.
"At first, there was a fight that broke out between two individuals right next to the bar," said Heidi Wittwer, who was at the bar at the time of the shooting.
The sheriff said the person responsible may have been told to leave the bar, and then returned to go on the shooting spree.
"One person got removed from the establishment and possibly came back a short time later," said Sheriff David Beth.
Wittwer said the bar was packed with dozens of people when the first round of shots were fired.
"We were told to duck, and then everyone started running and we all just ran very fast," Wittwer said.
Peter Ploskee woke up to the sound of gunshots near his home.
"Next thing you know, I hear a bunch of shots going off. [I] got up, ran out and looked out the window. Just see people running from the bar in every direction," he said.
"I didn't see nobody. Like I said, when I looked out the window, it was just chaos. People running every direction possible, so you know, who's who? Is that the guy? Is this the guy? People are just running, people screaming," Ploskee said.
Kenosha officials believe the incident was targeted and that the shooter may have known the victims.
"We don't believe this was a random act," Sheriff Beth said. "I currently believe the gunman knew who the victims were."
Sheriff Beth is expected to hold a press conference Monday afternoon to provide the latest details in the arrest and investigation.
What we know about the victims
One victim's brother and girlfriend said the Kenosha father had so much more life ahead of him.
Cartier Gaston drove for hours from Detroit after finding out his youngest brother, Cedrick, was one of the three killed.
The two other men killed, Stevenson and Donaldson, were also from Kenosha, according to the sheriff.
A 26-year-old Kenosha man, and two men, 22 and 23, from Wonder Lake, Illinois were the others who were wounded in the shooting.
Victims' loved ones hosted a vigil Sunday night, including Cedrick's girlfriend Veronica Scalia, as they hold on to one another and the memories of those lost.
"He would light up your day no matter what. When he walked into the bar there was smiles, everything," Scalia said.
The bar is known as gathering place for the Carthage College community. The campus was on lockdown for a period of time Sunday morning, according to the college website, but the college received a message at 6:20 a.m. from law enforcement that said the lockdown can be lifted.