Citibank can close your account and hold your money for 30-60 days without cause
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This is the truth, and I imagine it applies to just about every bank. Years ago, a friend of mine had this happen to her with USAA. It took an Executive Email Carpet Bomb to get resolved, and I haven't banked with USAA since.
I can absolutely recommend having a few accounts at different institutions, preferably at local or regional credit unions. I'm sure they have this same right to cease business with a customer, but I think it would be easier to resolve (if it happens in the first place). But by having multiple accounts, you can avoid losing access to the total sum of your money all at once, which is what happened to my friend.
In the UK, an individual can be prosecuted for “tipping off” someone about a SAR. I assume something similar exists in the US. This is why the banks respond with absolutely minimal communication.
Which is like the dumbest thing I've ever heard of. Money launderers surely have noooooo idea what's happening when their accounts get mysteriously locked and every rep they speak to is suddenly a powerless idiot.
This is working on the assumption that the majority of these flags end up being false positives (hence, "suspicious" activity, not "illegal"). If the common cases can be sorted out by a 30-minute phone call and faxing over some missing documentation, that saves resources on the law-enforcement side trying to reconstruct the same facts from "outside".
Of course, as you said, the real criminals will presumably take both the "bank suddenly started acting weird" and "here's an actual notice of investigation" as similarly strong signals to cut and run.
It's a non-story.