CHICAGO (CBS) – Police say a mob of teens is responsible for robbing and pushing people on Chicago’s Magnificent Mile Saturday night.

Tuesday, 2nd Ward Alderman Brian Hopkins said warning systems failed, catching police by surprise.

CBS 2’s Roseanne Tellez investigates what went wrong with the warning system.

“We heard the conductor of the train yelling at them, saying the police are coming,” said a witness, riding the Red Line during the Saturday night incident.

The red line rider, who does not want to be identified, gave CBS 2 News pictures and video of police questioning youth on a train Saturday night.

“Do your parents know where you are?” questioned police.

A fellow rider told that officer what was happening in the train station.

“They were running down the stairs. They were moving people, like pushing people out of the way, when they got to the turnstile, they just hopped over,” said the witness.

As many as 150 youth hopped off at the Chicago red line stop. According to Alderman Hopkins, it caused major chaos between there and the Water Tower.

“They started immediately making a lot of noise, shouting and yelling, pushing pedestrians,” said Hopkins. “So clearly this was designed to frighten people and intimidate people.”

Hopkins says some of the young people stole cell phones and purses, and says with so many cameras in the area, police should have seen what was about to happen that night.

“We’re going to do some internal review and figure out what went wrong so that the patrol officers aren’t caught with no warning like they were Saturday night,” said Hopkins. “They were vastly outnumbered by a significant margin.”

Chicago Police confirm only one arrest was made for a felony assault to a police officer. They say someone threw a cup of liquid at an officer.

Alderman Hopkins says police can’t prevent incidents like this one from happening, but they can respond more effectively.