Updated Hackers claim to have stolen all T-Mobile US's corporate data, customer accounts, network infrastructure - the whole lot.
According to emails from "pwnmobile": "We have everything, their databases, confidental documents, scripts and programs from their servers, financial documents up to 2009."
The mail, sent to The Reg, claims that the group tried to sell the data to T-Mobile's competitors, but was turned down. It is now offering it for sale to the highest bidder.
The message said the group tried contacting people by email. But given spam filters, and the weekend timing of the leak, it could be the messages never got through.
The mail contains some details of what has been stolen, and is available from the insecure.org list here.
We have contacted T-Mobile and will update you when we get any more information from them.
It is not clear yet whether this is a genuine breach or a blackmail attempt - there is evidence for both at the time of writing.
T-Mobile has 148m subscribers worldwide and 33m in the US.
A T-Mobile spokesman said: "The protection of our customers' information, and the safety and security of our systems, is absolutely paramount at T-Mobile. Regarding the recent claim, we are fully investigating the matter. As is our standard practice, if there is any evidence that customer information has been compromised, we would inform those affected as soon as possible." ®