A domestic dispute on board a Chicago subway train turned deadly Thursday afternoon when a man suddenly and repeatedly stabbed his girlfriend.

Authorities said the attack happened on board a Red Line train on the city's South Side at around 12:30 p.m. local time. Investigators said the victim was 25 years old, but did not immediately identify her further.

Chicago Police First Deputy Superintendent John Escalante told reporters that witnesses described hearing "some talk about a child" between the couple. Escalante said witnesses observed the man asking the woman a question. After she responded "no," he stabbed her and slit her throat.

Other passengers called 911 to report the murder. The unidentified suspect turned himself into a police officer after getting off the train at the next stop.

"This is domestic in nature,” Escalante said. “We’re sure of that."

The Red Line is the Chicago Transit Authority's busiest L line. The attack and the police investigation shut down one station for nearly two hours.

The Associated Press contributed to this report.

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