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.

A Philadelphia police officer shot and killed a 47-year-old man in the Cobbs Creek section of the city Sunday evening, police said.Police identified the man as 47-year-old Jonathan Corbin in an update Tuesday afternoon.

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.

An officer on routine patrol noticed the plaque was missing from its location on the 2000 block of South Broad Street Tuesday night.