People posted to social media that they were locked down inside stores at the mall for the reported shooting. Outside the mall, police cars, fire trucks and ambulances seemed to surround just about every exit.
WATCH: Latest police update on Oakbrook Center shooting
Oak Brook police said a security officer working the holiday detail at Nordstrom heard gunshots shortly after 6 p.m. and immediately called in the incident.
Police said two people were exchanging gunfire with each other in a corridor area in the east part of the mall adjacent to the Nordstrom.
"It was crazy," said Camila Reyes, shopper and witness. "It was a lot of shooting. It was a lot of shooting, it was so loud. I was in Nordstrom but I ran; I was getting out of the store and just ran to another store."
Four people were injured by gunfire, police said. One of those people is believed to be one of the suspects involved in the shootout. Police said they believe the other three were also struck by gunfire.
WATCH: Witnesses describe shooting at Oakbrook Center
Police cleared the mall store by store, investigating the possibility that a third suspect may still be at large. Police said he was wearing a blue puffy coat.
Police said the suspected shooter is a man in his 30s, who was struck multiple times and underwent several surgeries.
The other three shooting victims are two women in their 40s and one woman in her 20s. Each of them were struck once.
All gunshot victims have non-life threatening injuries, police said.
Additionally, a woman in her 20s twisted their ankle fleeing the shooting and was taken to be treated for a fractured ankle.
"Everybody was panicking," said Diane Sprague, witness. "We tried to keep everyone calm and safe: stay away from the windows. We locked the doors, stayed away from the back door. We gave people some water to calm people down. There were children, elderly people. Made sure everybody was not injured."
Another man was taken into custody after running into the Nordstrom and attempting to flee, police said. Police said they believe he was the companion to one of them involved in the shootout.
READ MORE | Oak Brook mall retail theft: Grab-and-run thieves target Nordstrom twice in one day, police say
Two weapons were recovered. Both are pistols, police said, including a 9mm.
Approximately 100 officers from various area law enforcement agencies responded to the mall Thursday night.
Despite the initial panic of the shooting, the pervasive mood of the scene after the shooting was calm, with police working methodically and shoppers generally waiting patiently for their turn to leave.
DuPage County board member Pete DiCianni said he was shopping nearby at the time of the shooting and saw the police response. He said he was told by his family who are locked down at the mall that the shooting happened just outside the Nordstrom store at the mall.
Witnesses said the mall was crowded with last minute Christmas shoppers at the time of the shooting.
Family members rushed to the mall to meet their loved ones as soon as the lockdown is lifted, though police have not said how long that will take.
The last time there was a shooting at Oakbrook Center was nine years ago, Oak Brook police said, when a security officer opened fire during a confrontation.