One source would be O.K. if the claim was not extraordinary. By inserting this claim into the article you have turned the victims of the massacre into anti-Polish collaborators who were punished for doing that by the Nazis and their Ukrainian cooperators. This claims changes the whole prospective of the events and it is VERY VERY serious allegation therefore you need to back it up with some better sources. One PDF article in Ukrainian language in far less from satisfactory. Thanks.