Do you know this person? It's hard to tell if you have been victimized by a professional con artist or if this is just a bad friend/family member/acquaintance? Do you know where this person lives?

I had an experience over a decade ago where I paid with a check and the cashier typed my account and routing number into a credit card machine and the bank processed it. Then when I got the statement, they had also deposited the paper check. It was a simple error on the part of the store and I worked it out with them.

What was really disconcerting to me was that when I called the bank and asked why they would process the same check number, for the same amount twice for the same retailer, the service rep got snippy with me and said that they didn't know whether it was my mistake or the retailers and said I needed to talk to the retailer.

To bring this back to your question, that experience makes me believe that this is probably easier to do than most people think. Yes, an account would be needed with the bank but if it's a sophisticated operation, they may have gotten control over one the bank trusts. Whether you should worry depends on a lot of things one being the country you live in. If this is a personal account in the US under 100K, you are insured by the FDIC. It might take a while to sort out if they drain your account and you probably have bills to pay so it will at least be a major annoyance. If this is a business account, the bank is under no obligation to refund your money and there is no federal insurance. If you are in this situation you should get an account with 2-factor security.

Regardless of what kind of account it is, you should watch it like a hawk and if there are any odd transactions, contact the bank immediately. I would also call the bank and tell them of your concerns. Ask them to not allow any transfers without your explicit authorization. If they say they can't, you might want to look for a new bank.