A 21-year-old man who was riding a CTA bus through Park Manor late Sunday was in critical condition after he was shot in the chest by another man who tried to rob him, leading to an exchange of gunfire on the bus, according to Chicago police.

       Officials said both the 21-year-old and the man who allegedly tried to rob him were armed with handguns and neither had a firearms owner identification card, or FOID card, nor a license to carry a concealed weapon.

       Officers were called to the first block of East 71st Street in Park Manor around 10:40 p.m., where they learned there had been a shootout on the CTA bus that had been headed east.

       While riding the bus, the 21-year-old was approached by a man he didn’t know who “attempted to rob him at gunpoint,” according to a police media notification.

       But the 21-year-old also was armed and he pulled out his own handgun and began firing at the would-be thief, police said. The other man fired his gun as well, striking the 21-year-old in the chest, officials said.

       Both men exited the bus in the Park Manor neighborhood and the wounded 21-year-old was taken by Chicago Fire Department paramedics to University of Chicago Medical Center, where he was listed in critical condition.

       The alleged shooter, whose name and age hadn’t been released as of Monday morning, was located by officers in the area of East 72nd Street and South Michigan Avenue, according to authorities. He was placed under arrest and taken to a police station for questioning.

       No one else was injured during the shootout. It wasn’t immediately clear whether anyone else was on the bus at the time.

       Buses in the area were rerouted for at least an hour as police investigated, according to posts by the CTA on Twitter.

       Check back for updates.

       kdouglas@chicagotribune.com

       Twitter @312BreakingNews

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