Keith Fahrney, speaking exclusively with ABC News, said he was driving by when he saw a woman being stabbed and was then attacked himself.
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He said it wasn't immediately clear the woman was being attacked.
"I stopped and she kind of turned toward me, and her face was just bright red with blood," he recalled. "The lady just looked at me and said 'I need help,' so I turned back around to go to the Jeep. The guy was already inside my Jeep with the door shut, trying to take off with it."
That's when Fahrney said he grabbed suspect Christian Soto by the shirt and yanked him out of the car.
"I open up the door, grabbed him by the shirt and threw him down on the ground, right, and then he kind of went after me and I kind of tried to defend myself and the next thing I know...I felt these taps on the side of my head," Fahrney said.
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The two tussled in the street.
"He was hitting me in the head," Fahrney said. "I had no idea something was puncturing me in the head at the time."
At that point police were already racing to the scene, responding to multiple calls for help.
The Winnebago County State's Attorney J. Hanley also spoke about Fahrney's heroic actions.
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"Soto entered Keith Fahrney's Jeep and attempted to steal it," Hanley said. "Fahrney pulled Christian Soto from the Jeep, but again, was being attacked by the knife. It was at this time that police arrived.
SEE ALSO: Court records detail Rockford stabbing attack suspect Christian Soto's past police contacts
Four people were killed in the attack and seven others injured. The four victims killed were identified as Jenna Newcomb, 15; Jacob Schupbach, 23; Jay Larson, 49; and Romona Schupbach, 63.
Soto, 22, has been charged with multiple offenses, including murder.
"I really didn't know what was going until the cops were telling me 'Thank you, thank you for getting this guy,'" Fahrneysaid.