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A CTA employee was in good condition Tuesday after being attacked on a 71st/South Shore route bus in the city’s Grand Crossing neighborhood, according to Chicago police and CTA.

The employee, a 44-year-old instructor who was training a bus operator, was involved in an argument with a female rider who boarded the eastbound bus, which was near the 400 block of East 71st Street, around 11 a.m. without paying, authorities said.

The argument escalated and the rider struck the employee several times with a fist, police said. The rider then pulled the employee off the bus, continued to strike her and eventually fled, according to Chicago police.

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The Chicago Fire Department treated the employee, who suffered abrasions and was later taken to South Shore Hospital, police said.

“We are outraged by this senseless act of violence against a CTA employee who is dedicated to keeping Chicago’s transit system running. Such attacks are not only unacceptable, they are also a felony,” said CTA President Dorval R. Carter, Jr. in a statement. “CTA is working closely with Chicago Police to aggressively pursue this case as a top priority. We will utilize all the resources at our disposal to find the perpetrators and hold them accountable to the fullest extent of the law. CTA’s transit workers are dedicated public servants and deserve our respect and protection.”

As of Tuesday evening, no one was in custody and detectives were investigating the incident, the Chicago Police Department said.