Once again, a Philadelphia police officer has been shot and killed by a criminal who should have been in prison instead of free to commit more mayhem and destruction.

Over the last two years, I've written more than a dozen columns about the murders committed by repeat offenders already convicted of a violent felony (sometimes murder) who spent little or no time in prison.

These columns always ask why those in the criminal system - judges, police, lawyers, probation officers, parole boards, prison officials and elected politicians - aren't held responsible for permitting such people to roam free. Unfortunately, nothing happens - except more people are murdered.

This time it was Philadelphia Police Sgt. Stephen Liczbinski, shot and killed responding to a bank robbery in Port Richmond.

All three suspects in the killing - Howard Cain, Levon T. Warner, Eric Floyd - have convictions for violent felonies.

Warner was sentenced in 1997 to 7 1/2 to 15 years on a robbery charge, one to 5 for possessing an instrument of crime and five to 10 for criminal conspiracy.

Cain was convicted in 1996 of four counts of robbery, carrying firearms without a license and criminal conspiracy. He was sentenced to five to 10 years for each robbery charge, two to four on the other charges. He had also been arrested for aggravated assault, carrying firearms without a license and reckless endangerment.

Floyd was sentenced to five to 10 years in 1995 for robbery and rearrested in 1999 for violating parole. He was released early, and convicted again in 2001 for two robberies in Lancaster.

Yet Mayor Nutter repeats the usual sophistry about guns. Hizzoner said, "That officer was assassinated on the streets of Philadelphia. There was nothing that could have protected him - that weapon penetrates vehicles."

His statement illustrates why our elected representatives are unable to reduce violent crime.

The mayor's lack of knowledge of weaponry notwithstanding, there is one patently obvious policy that definitely would have protected the officer.

If Levon Warner had served his full sentence, he would've been in prison until 2012. He could not have committed any crime in 2008.

If Howard Cain had served his full sentence, he would've been in prison to 2052. He would not have murdered anyone in 2008.

If Eric Floyd had served his full sentence, he'd have been in jail, not robbing banks, in 2008.

But all three served less than the max and committed more violent crime. This time a cop ended up dead. Why isn't the mayor addressing this more easily remedied and more salient issue?

The man who pulled the trigger should have been in prison - it's that simple. All the unconstitutional gun laws that City Council passes and the mayor signs wouldn't have prevented men like this from robbing that bank and killing Liczbinski.

The only thing that would've prevented this homicide was the one thing politicians, judges, prison officials in Philadelphia don't want to address. Warner, Cain and Floyd should have been behind bars at the time they were committing the robbery.

Tragically, this is not an isolated incident. Here are just three of many more examples:

* Jerome Whitaker, who shot Officer Mariano Santiago, had an arrest for a 1994 homicide. He served just 11 years before being paroled in July 2006. He was arrested about a year later for violating parole and released a few months later - only a few weeks before shooting Officer Santiago.

* Mustafa Ali killed two retired officers working as bank guards. Convicted in 1993 of robbing a bank, he was sentenced to only seven years, despite being eligible for 11 1/2, according to sentencing guidelines. Ultimately, Mustafa didn't serve even the seven years of the plea-bargained sentence.

* Solomon Montgomery, who killed Officer Gary Skerski, had a record of violent crimes. He was also acquitted by a lenient Philadelphia judge after being arrested for shooting someone.

It's time to address the real issue: the incompetence, ineffectiveness and insensibility of a system that doesn't seriously incapacitate violent criminals. *

Michael P. Tremoglie is a former Philly cop and the author of "A Sense of Duty," available at Barnesandnoble.com.