CHICAGO (CBS) — Violent crime on CTA trains has increased over the past five years, yet the city often boasts about its security upgrades, including a multi-million dollar network of thousands of cameras. How helpful are the images from those cameras?

CBS 2 Investigator Dorothy Tucker tries to paint a clearer picture.

Time and time again, the city holds press conferences to brag about cameras on the CTA; a network of approximately 32,000 cameras on board trains and buses, and at bus and rail stations.

Those cameras didn’t stop a masked gunman from opening fire in the tunnel connecting the Blue Line to the Red Line at Jackson on Dec. 6 during the height of the evening rush.

Although police said the gunman was caught on camera, two months later there have been no arrests, and images of the shooter have not been released. Police would only say “each case is different.”

Security consultant Ross Rice, a former spokesman for the FBI in Chicago, knows a thing or two about law enforcement and the media, so we asked him under what circumstances police might sit on images of a suspect.

“I would say only in a case where you have a very good idea of who the suspect is,” he said. “Because they’re watching Channel 2 news, the suspects, and if they see their picture on the news, what are they going to think? ‘Oh, they’re after me,’ and they’re going to flee, and that prolongs the investigation, and it makes it much harder to solve.”

That clearly wasn’t a concern on Feb. 6, when police released a community alert regarding a robbery at the Sedgwick stop on the Brown Line two days earlier, including images of the robber wearing a fur boa.

On Dec. 7, police also issued a community alert about a Red Line robbery two weeks earlier, with clear images of two suspects.

Rice said law enforcement officials only release such images “when it serves a law enforcement purpose.”

“We always need the public but there are times we need the public more than others, and this gets back to the balancing act,” he said.

Two months after the CTA tunnel shooting, Rice offered up another possibility regarding the unreleased surveillance pictures.

“Then you go to those cameras and they didn’t record, the lens was dirty, it was a malfunction, and you’re stuck,” he said.

While surveillance pictures and video can be helpful to catch and convict a criminal, the pretrial publicity also can prejudice a jury, so police prefer not to release such images if they can find a suspect without the public’s help.

Dorothy Tucker