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CHICAGO — A shooting outside Wendell Phillips Academy High School on the South Side injured a girl and a security guard Tuesday afternoon.

Chicago police said the shooting injured a 14-year-old girl. Ambulances rushed the teenager to Comer Children’s Hospital in serious condition. Authorities added that the 45-year-old wounded security guard arrived in serious condition at the University of Chicago Medical Center.

Police later said the security guard was in fair condition.

About 3:30 p.m., Chicago first responders arrived in the 200 block of E. 39th Street.

A preliminary investigation revealed the gunman waited outside school doors around dismissal time, police said in a briefing. The shooter allegedly opened fire once the security guard opened school doors, according to police.

“Immediately, there was an offender in the street area who started shooting in the security guard’s direction,” CPD Chief of Detectives Brendan Deenihan said. “He struck the security guard several times.”

WATCH: Authorities speak after 2 shot outside Wendell Phillips Academy HS

The bullets pierced a double door, striking the 14-year-old girl, police added.

Police confirmed what one student who was inside the school told WGN. The student revealed that the shooter was outside the school when gunfire erupted.

Wendell Phillips Academy High School parent Michelline Joseph called the incident an “unfortunate situation.”

“Now it’s going to call for an even heavier police presence, I’m hoping, at the school to monitor these kids,” Joseph said. “Kids should never have to go to school under that type of environment and situation.”

Diana Cooper is a school staffer at Wendell Phillips Academy. She also has a daughter who is a student at the Bronzeville-area school. She told WGN she believes Tuesday’s shooting links back to an incident on Friday.

“I heard it Friday that they jumped on a young man from one of the co-workers,” Cooper said. “And he threatened to come back.”

Police said the offender ran away from the scene and may have gotten inside a vehicle. A motive is still unknown.