T-Mobile USA is investigating a claim that a hacker has broken into its databases and stolen customer and company information.
The claim was posted on the security mailing list Full Disclosure by someone using the handle "pwnmobile".
"We have everything, their databases, confidential documents, scripts and programs from their servers, financial documents up to 2009," the posting said.
The hacker said the information had been offered to T-Mobile's competitors without success, so now the information was on offer to the highest bidder.
T-Mobile issued a statement that the company is investigating the matter and will inform customers as soon as there is evidence that information has been compromised.
Security experts have said although the claim appears to be legitimate at first glance, such a public offer suggests that it is a hoax, according to US media reports.