CHICAGO (CBS) — Chicago Police say the man in surveillance pictures is responsible for an attack at a CTA Blue Line station.

An alert commuter may have spotted him last night and tipped off the police.

CBS 2’s Jeremy Ross reports from Logan Square.

That tipster says after contacting the CTA and 911, he stopped inside a nearby police station.

Tonight  he remains frustrated.

That as police continue their search for the attacker.

Chicago Police put out a warning and surveillance video.

They were searching for a man accused of posing as an undercover cop Tuesday.

The person is accused of snatching a 70-year-old man’s wallet.

He allegedly hit the victim who was seen leaving the blue line bloody.

CPD identifying the attacker with some unusual markings on his hands.

“Something struck me as kinda familiar about this guy,” says Mike Ramsey who was riding the Blue Line late Thursday night.

He carefully snapped pictures of someone acting suspicious.

And compared the images, including the discoloration on the suspect’s hands to the police bulletin he had seen earlier.

Ramsey pushed the emergency train button.

The train eventually it stopped.

CTA looked into Ramsey’s concerns as he showed them the pictures.

But after about five minutes, the man left.

And Ramsey followed

“I said ‘hey why did you beat that guy up at the CTA station?’ And he said ‘Idon’t know what you’re talking about.'”

Eventually the alleged attacker began to run.

Ramsey called 911 shortly after,  but the man with the distinctive hands slipped through law enforcement’s fingers.

After speaking with police, Ramsey was advised to call 911 first.

He says he’ll certainly do that going forward.

Chicago Police are looking into the matter.

CTA says its personnel followed the appropriate protocol.