Free online collaborative encyclopedia Wikipedia has imposed a ten-day block on edits from computers in US Congress building.
The block is a response to anonymous changes to Wikipedia entries on politicians, businesses and historical events.
The entry for the assassination of John F Kennedy was changed to say Lee Harvey Oswald was acting "on behalf of the regime of Fidel Castro", according to the BBC.
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The entry on the moon landing conspiracy theories was changed to say they were "promoted by the Cuban government".
The rogue edits were revealed by a Twitter feed, @congressedits, which posts every change made from a US government IP address.
Wikipedia allows any user to make changes to entries, but the changes are policed by volunteers who can remove inappropriate content and impose bans on users who flout editing rules.
Wikipedia monitors have been warning editors from the House of Representatives since March 2012, and imposed a one-day block on edits from the US Congress building earlier this month.
Blocks on edits from computers in the building have been imposed before following acts of vandalism.
Jimmy Wales, the founder of Wikipedia, told the BBC the incident did not surprise him, and vandalism has "always gone on and it always will".
Earlier this year, the BBC discovered that an edit from a UK government computer added the phrase "all Muslims are terrorists" to a Wikipedia entry about veils.
The incident followed a report by the Liverpool Echo which found insults had been added by a government computer in Whitehall to the Wikipedia entry for the Hillsborough disaster.
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