Police are appealing for information after paint was daubed on a memorial to the 10 men murdered by the IRA in the 1976 Kingsmills massacre.
The criminal damage took place in the Kingsmill Road area of Bessbrook, Co Armagh, around 4.15pm on February 7.
Police are treating the attack on the memorial as a hate crime.
Inspector Philip McCullough said: “Police are treating this as a hate crime and are appealing to anyone who witnessed the incident or with information to contact officers at Ardmore Police Station on the non-emergency number 101.
“Or if someone would prefer to provide information without giving their details, they can contact the independent charity Crimestoppers and speak to them anonymously on 0800 555 111.”
UUP MLA Danny Kennedy said it was a “sickening attack”.
SDLP Newry and Armagh MLA Dominic Bradley said he was “disgusted” at the attack.