(CNN Investigations) Aviation police officers in Chicago are sitting targets for potential terrorists because they don’t carry guns and are told to run and hide in the event of an attack, the union representing the officers said Thursday.

Matt Brandon, the secretary-treasurer of SEIU Local 73, warned city officials that the terrorist attack in Turkey is a “clear signal from international terrorists that they intend to kill and maim people in airports across the world.”

In a scathing letter to Aviation Department Commissioner Ginger Evans, Brandon wrote, “Imagine if you will, the video recorded by cameras in Chicago’s airports showing aviation police officers running alongside panicked and helpless travelers with a suicide vested terrorist pursuing them intent on killing as many of them as possible.”

A CNN investigation in December revealed that the nearly 300 aviation police officers at O’Hare and Midway airports are not only unarmed, but they are instructed to “run and hide” in the event of an active shooter. Officers repeatedly told CNN that the policy puts them in danger.

The aviation police officers work for the Chicago Department of Aviation and are all certified law enforcement officers in the state of Illinois. Many also work in suburban police departments or are military veterans.

Working alongside those officers is the Chicago Police Department, the primary law enforcement agency at both airports, with 231 budgeted armed officers and supervisors. The Chicago police officers carry guns and and the police department has maintained that its staffing is sufficient at both airports.

“I shudder to contemplate what hollow words and senseless explanation you would offer the families of the victims when they asked the inevitable questions, ‘Why were the officers running away? Why didn’t they try to stop this?”’ Brandon wrote.

He added that “by arming the aviation police officers you’ll never see them running away on a video.”

The letter was sent Thursday to Evans and copied to Mayor Rahm Emanuel and other city officials.

The Chicago Department of Aviation has consistently maintained that a multi-faceted security plan at O’Hare and Midway is most effective and that there are sufficient number of armed police at the two airports.

In a statement sent to CNN, the aviation department said:

“The Chicago police department is the primary law enforcement agency at O’Hare and Midway International airports. Chicago police provide the command structure, armed officers and specialized teams for explosives and active shooter response and work in close coordination with the FBI, TSA, US Customs and Border Control, aviation security officers, otherwise known as ASOs, and others to implement a multi-layered approach for protecting the airports.

“CPD’s armed officers also work with ASOs to monitor and control access at the secured area, perimeter gates and respond to terminal door alarms. The security agencies protecting our airports are in constant communication with our federal partners and make adjustments as needed.”

In the wake of the CNN investigation, the aviation department’s police chief resigned in April and a city alderman has proposed an ordinance that would arm the officers.

“We have a security risk,” said Alderman Chris Taliaferro, whose proposal has not yet been put on the public safety committee’s agenda.

Taliaferro, an attorney and former veteran Chicago police officer, said it made no sense for trained law enforcement officers not to carry a gun while on duty.