The attorney did not disclose the subject of the investigation to Qwest, though Qwest pledged to co-operate with the probe, according to a statement issued yesterday.
No one at Qwest or the US Attorney's office was immediately available for comment.
News of an investigation by both the US Attorney in Colorado and by the FBI was first reported in the Wall Street Journal earlier this month. At the time of the report, Qwest denied any knowledge of such investigations.
The company is already being investigated by the US Securities and Exchange Commission over its accounting practices, including how it recognised revenue and sales of optical capacity and other matters relating to the sale of equipment to customers who had purchased other services from Qwest. That investigation began in April.
KPNQwest, the company's European joint venture with Koninklijke KPN, filed for bankruptcy in late May.