CHICAGO (CBS) -- Chicago police are asking for the public's help to identify a suspect in a robbery that took place Monday on the CTA Green Line train.

Police said the suspect robbed a woman on the train traveling westbound from Morgan to Ashland. The incident happened in the 1600 block of W. Lake Street around 6:50 p.m. on Monday.

The unknown suspect approached the woman and demanded money or he would shoot her, implying he had a firearm.

The suspect then fled the scene.

Police only gave a vague description of the suspect but released images of him captured on CTA security cameras.

Anyone with information is asked to contact the Bureau of Detectives, Area Central at 312-745-4706.