Surveillance image of the man who wanted for pretending to be an undercover police officer before assaulting and robbing a 70-year-old man March 6 at the Monroe CTA Blue Line station. | Chicago Police

A man pretending to be an undercover police officer assaulted and robbed a 70-year-old man at a CTA Blue Line station early Tuesday in the Loop.

The man was waiting for a train at 2:57 a.m. at the Monroe Blue Line station, 114 S. Dearborn, when the suspect walked up and asked to see the man’s ID, claiming to be an undercover police officer, Chicago Police said.

When the man took out his wallet, the suspect tried to grab it from him, police said. When the man resisted, the robber punched him in the eye and ran away with his wallet.

The man was taken to Northwestern Memorial Hospital, where his condition has been stabilized, police said.

Investigators released surveillance images of the suspect Tuesday afternoon and asked the public for help identifying him.

He was described as a man between 50 and 55 years old, standing between 6 feet and 6-foot-2 and weighing 180-220 pounds, police said. He was wearing a gray knit cap, a black puffy coat, blue jeans and tan boots and was carrying a black garment bag.

Anyone with information about the robbery was asked to call Area Central detectives at (312) 747-8384.

Surveillance images of the man who wanted for pretending to be an undercover police officer before assaulting and robbing a 70-year-old man March 6 at the Monroe CTA Blue Line station. | Chicago Police