CHICAGO (CBS) — Eighteen uniformed Chicago police officers were headed to Springfield on Thursday to attend parole hearings for three convicted cop killers.

WBBM Newsradio’s Bernie Tafoya reports the officers planned to present a silent but visible opposition to early release for three men serving lengthy prison terms for killing police officers in the 1970s.

In 1973, 36-year-old tactical officer Edward Barron was trying to stop a man who matched the description of a suspect from a recent robbery, when Joseph Bigsby shot Barron in the head, killing him.

Barron, now 57, was sentenced to 200 years in prison, and has previously been denied parole.

In 1970, Sgt. James Severin and Officer Anthony Rizzato were shot and killed while walking across a ballfield at the Cabrini-Green public housing complex.

“Johnnie Veal and George Knight shot at them with .30-30 hunting rifles from the upper floors of one of the Cabrini-Green housing projects,” said Chicago Police Sgt. Frank Iglinski.

Veal and Knight were convicted of killing Severin and Rizzato to seal a deal between two rival gangs were sentenced to 100 to 200 years in prison.

“These guys were just trying to protect the citizens of Cabrini-Green, and make them feel safer, and really just gunned down because of the uniform they wore,” Iglinski said.

Iglinski said Bigsby, Veal, and Knight should remain in prison for what they did.