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Qwest, with Dutch telco KPN, was part owner of the European KPNQwest network, which filed for bankruptcy at the end of May.
Banks that lent money to KPNQwest have questioned the accounts for the failed firm, while its auditors have said that accounts were not signed off.
Drake Tempest, Qwest executive vice-president and legal officer, said, "We have no reason to believe that we are the subject of any investigation by the US Department of Justice.
"It's outrageous that we would learn about such an investigation through the media. We have disclosed everything asked of us and have co-operated fully with the Securities Exchange Commission and Congress," Tempest added.