Not true ....
The law says .....
In 2001, the chief national bank regulator, the Office of the Comptroller of the Currency, which regulates all banks with national in their name, warned banks that the burden of proof in a reinvestigation is on the bank to show that a transaction was authorized (in other words, the bank isn't supposed to presume the consumer is guilty, but innoncent, when the consumer claims fraud).
I did not say that the bank would not accept your word that a transaction was fraudulent as sufficient evidence in most cases. Nonetheless, the merchant who processed the transaction also has a right to due process -- hence the allowance of ten days to conduct an investigtion. Note that the bank may well ask you for further proof or find a transaction to be legitimate if the merchant furnishes supporting documentation with your digitized signature cut and pasted from a legitimate transaction, for example.
The institution is generally required to determine whether
an error occurred within 10 business days and report to
the consumer within 3 business days of completing the
That provision allows up to thirteen business days before the bank mails its decision, during which the money is not in your account. Also, note that this does not count the time from the actual fraudulent charge until you got your bill and reported it, which could be well over a month, when your account also was short.