Fallen officer Charles Knox was honored with a plaque near where he was killed in 1992. The plaque appears to have been removed, police say.

PHILADELPHIA (WPVI) -- A plaque dedicated to a fallen Philadelphia police officer has gone missing, and now investigators are trying to figure out what happened to it. The plaque was placed in memory of Ofc. Charles Knox, who was killed in the line of duty during the summer of 1992.

Charles Knox, 31, was shot and killed the night of Aug. 30, 1992, while responding to an armed robbery at a Roy Rogers restaurant at South Broad Street and Snyder Avenue.

Investigators are searching for a missing plaque dedicated to a fallen Philadelphia officer who was shot to death during an armed robbery over 30 years ago.

The ceremony was designed to remind the people of Philadelphia that these men and women are willing to put their lives on the line every day they show up for work.

The Philadelphia Police Department is asking for help in finding a missing memorial plaque that honors fallen Officer Charles Knox, who was killed in the line of duty in 1992.