Online reference site Wikipedia blames US Congress staff for partisan changes to a number of political biographies.
Computers traced to Capitol Hill removed unpalatable facts from articles on senators, while other entries were "vandalised", the site said.
An inquiry was launched after staff for Democratic representative Marty Meehan admitted polishing his biography.
Wikipedia is produced by readers who add entries and edit any page, and has become a widely-used reference tool.
'Liberal' to 'activist'
Using the public history of edits on Wikipedia, researchers collected the internet protocol numbers of computers linked to the US Senate and tracked the changes made to online pages.
The site lists half a dozen prominent biographies that had been changed by Senate computers, including those of Minnesota Senator Norm Coleman, California Senator Dianne Feinstein and Senator Tom Harkin of Iowa.
Senator Coleman's office has confirmed that staff there had made a number of changes to his online record.
Where he was described as a "liberal" back in college, this was changed to "activist".
Among other changes, staff also deleted a reference to Mr Coleman voting with President Bush 98% of the time in 2003, despite running as a moderate the year before.
Wikipedia said staffers of Senator Tom Harkin had removed a paragraph relating to Mr Harkin's having falsely claimed to have flown combat missions over North Vietnam, and his subsequent recantation.
A handful of miscellaneous vandalism edits had been made to some senators' articles, it said.
One example was the entry for Republican Senator Tom Coburn, of Oklahoma, who it was falsely alleged had been voted "most annoying senator".
Bush editing block
Senator Coleman's chief of staff, Erich Mische, said editing was done to correct inaccuracies and delete information that was not reflective of the politician.
"They've got an edit provision on there for the sake of editing when things are not accurate," Mr Mische told the Associated Press.
"I presume that if they did not want people to edit, they wouldn't allow you to edit."
Wikipedia says the controversy raises questions about whether it is ethical for those with a vested interest in the subject to edit entries about it.
It said the Congressional computer network has been blocked from editing for brief periods on a number of occasions in the last six months due to the inappropriate contributions.
The article on President Bush has been altered so many times - not just from within Congress - that Wikipedia's volunteer monitors have had to block further "editing".
But it also says its investigation showed the vast majority of edits from Senate IPs were "beneficial and helpful".
Massachusetts newspapers disclosed last month that staffers for Representative Marty Meehan had polished the boss's Wikipedia biography.
Deleted were references to a long-abandoned promise to serve only four terms, and to his campaign war chest.
Wikipedia was founded in 2001 and has since grown to more than 1.8 million articles in 200 languages. Some 800,000 entries are in English.
It is based on wikis, open-source software which lets anyone fiddle with a webpage. Anyone reading a subject entry can disagree, edit, add, delete, or replace the entry.
A December 2005 study by the British journal Nature found it was about as accurate on science as the Encyclopaedia Britannica.
But it has been criticised for the correctness of entries, most recently over the biography of prominent US journalist John Seigenthaler - which incorrectly linked him to the Kennedy assassinations.
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