Johnson was troubled after realising the clip had been watched by 40 people
At least five males have been implicated in the alleged sexual assault after the teen disappeared on Sunday.
But Chicago police only learned of the attack after the girl's distraught mother confronted police superintendent Eddie Johnson with screen grabs of the alleged sex attack.
Detectives were immediately ordered to investigate by Superintendent Johnson, who was said to be "visibly upset" after watching the video.
It is the second time in three months Chicago police have had to investigate a live streamed attack
The force also called on Facebook to remove the video, which they quickly did.
At the time Mother Stacey Elkins said: "I saw her face and the look on her face was just pure fear. And I, I just want my daughter home."
The teenager has since been reunited with her mother two days after her disappearance on Sunday afternoon in Chicago's Lawndale district.
The senior officer is believed to have been particularly troubled after realising the video had "40 or so live viewers and no one thought to call authorities", a police spokesman has claimed.
The 15-year-old went missing on Sunday afternoon
Detectives are now looking for five or six males in connection with the incident – and are interviewing the girl.
The case is being treated as a sexual assault.
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It is the second time in three months the Chicago police have had to investigate an attack streamed on Facebook live.
In January police apprehended four people after a live stream video showed them allegedly physically abusing a disabled man.
Police are on the look out for five or six males
Jordan Hill, Tesfaye Cooper, and sisters Brittany and Tanishia Covington were each charged with aggravated kidnapping, hate crime, aggravated unlawful restraint, and aggravated battery with a deadly weapon.
Tanisha Covington was the eldest at 24, while her sister and the two men are 18 years old.
The incident drew the attention of then-President Barack Obama, who called it "terrible" in an interview with Chicago's ABC-TV affiliate.
He said: "Part of what technology allows us to see now is the terrible toll that racism and discrimination and hate takes on families and communities"