The LaSalle Blue Line station platform. | CTA

An armed citizen held an alleged robbery offender at gunpoint following a violent mugging on a CTA train in the Loop on Wednesday morning, police said. Charges are pending against the offender, and the good Samaritan is also in legal jeopardy.

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Officers responded to the LaSalle Blue Line station at 6:50 a.m. after 911 callers reported that a man beat and robbed a passenger while riding a train, police said. Some callers reported that the robbery offender was lying on the platform with a private citizen holding him at gunpoint.

The alleged robber was detained by arriving cops.

A 29-year-old man told police the offender lunged at him as he sat on a Blue Line train minutes earlier. The offender began punching the victim in the head and face, then took unspecified property from the man before exiting the train car as it stopped at LaSalle, police said.

A private citizen intercepted the offender on the platform and held him at gunpoint until police arrived, a source confirmed.

The source also said the private citizen holds a valid Firearm Owner’s ID card.

Charges are pending against the offender, police said. The victim declined medical treatment, and officers returned the man’s stolen property.

As of midday Wednesday, prosecutors in the Cook County State’s Attorney’s Office continued to mull the possibility of filing weapons charges against the citizen, a source said.

Among other possibilities, the good Samaritan could face charges for possessing a firearm on Chicago Transit Authority property.