Graffiti scribbled on a 19th Century fort in West Sussex is being treated as an "aggravated heritage crime".
The Scheduled Ancient Monument at Shoreham-by-Sea's harbour was tagged on two sides, Sussex Police said.
It is believed to have happened sometime between 17:30 on Monday and 15:30 BST on Thursday.
PC Mike Harvey said: "Any crimes committed on the site which directly affect it are treated as aggravated heritage crimes."
Anyone recognising the graffiti is asked to contact officers.