I don't know about US law, but in Europe if you're a financial institution and you suspect a customer transaction is involved in money laundering or a similar i

I don't know about US law, but in Europe if you're a financial institution and you suspect a customer transaction is involved in money laundering or a similar it's a criminal offence to inform the customer without first clearing it with the relevant legal authorities. If there is some kind of investigation going on MtGox might be completely unaware of it, saying "there's a backlog" is a pretty common way to provide cover while such an investigation is on-going. One of the reasons you get fully licensed as a financial institution by regulators is that you get some level of legal protection/ring-fencing from the activities of your own customers. (IANAL but I used to work on real-world forex trading desk in a previous life, hence had to undergo the mandatory FSA training)