Twitter chief executive Jack Dorsey has told US law-makers that the company's algorithms have not always been "impartial".
He said the platform "unfairly" reduced the visibility of 600,000 accounts, including some members of congress.
But he was unable to immediately say whether a majority of them were Republican, Democratic or otherwise.
Mr Dorsey is being questioned by senators over allegations that Twitter censors conservative voices.
Several republicans, including US President Donald Trump, have accused Twitter of bias, which the company denies.
"Twitter does not use political ideology to make any decisions, whether related to ranking content on our service or how we enforce our rules," said Mr Dorsey.
However, he said the company had made mistakes.
On Wednesday, the US Department of Justice said it would investigate "growing concern that these companies may be hurting competition and intentionally stifling the free exchange of ideas on their platforms".
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