The Celtic League says a vandalism attack at Hango Hill, Castletown may have been politically motivated.
Those responsible painted slogans and obscene symbols near the execution site of Manx martyr Illiam Dhone.
In a statement, the League says although it's not clear whether the graffiti is supposed to be anti-nationalist, its timing is suspicious.
The annual Illiam Dhone commemoration is due to be held at the site on Monday (January 2), and the League says the vandalism seems deliberately timed.
The League says the way police have investigated this graffiti stands in sharp contrast to its probe into nationalist graffiti two years ago.
Director of Information Bernard Moffatt says in the past the chief constable threatened dire consequences for anyone caught painting nationalist slogans, but no message has been issued in this case.
He describes a police presence at the Hill to deter further attacks as 'locking the stable door after the horse has bolted'.
Mr Moffatt is also pointing out that if the attack was targeting nationalists it was in the wrong place, because the building at the hill, now in ruins, was built a century after Illiam Dhone died.
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