PHILADELPHIA (WPVI) — A plaque dedicated to a fallen Philadelphia police officer has gone missing, and now investigators are trying to find out what happened to it.

The plaque was placed in memory of Ofc. Charles Knox, who was killed in the line of duty in the summer of 1992.

It was installed at a sidewalk location on the 2000 block of South Broad Street in South Philadelphia. An officer noticed the plate was missing while on patrol Tuesday night.

It is not known when the plaque was removed. Investigators say the amount of dirt and debris in the area means the plaque may have been missing for some time.

Investigators checked with the Fraternal Order of Police and management at a nearby Walgreens store, but were unable to find anyone who gave permission for the plaque to be moved.

Police are now examining footage from Real-Time Crime Center cameras in the area for any clues.

“The Philadelphia Police Department is committed to preserving the memory of our fallen officers,” Police Commissioner Kevin J. Bethel said in a statement. “We are taking this matter very seriously and are asking anyone with information to come forward.”

If you can assist in the investigation, you are urged to contact the Southern Detective Division at 215-686-3013. You can also call or text the police tip line at 215-686-TIPS (8477).

Officer shot during armed robbery

Officer Knox was shot around 8:30 p.m. on August 30, 1992, while responding to an armed robbery at a Roy Rogers restaurant in the 1400 block of Snyder Avenue.

Knox entered the restaurant and saw a suspect holding a gun to the manager’s head. He ordered the suspect to drop the weapon, but instead the suspect lunged at him.

During the struggle, police say a second suspect came out of the manager’s office behind Knox, put a gun under his vest and fired.

Knox fell, and the suspect shot him again while he was on the ground.

Both suspects were sentenced to life in prison for killing Officer Knox.

He served with the Philadelphia Police Department for more than two years.