Chase closed my account and is asking me to confirm the validity of my deposits. How can they do this?

Chase closed my checking account without any explanation at the end of August. When I contacted my branch in person, they were told over the phone from the main offices that I would receive a check of the remaining balance (over 3k) and a letter in the mail. After not having heard anything for over a month, we headed back to the branch, where they were informed that our account is under investigation and I need to provide documentation for the validity of the funds that came into the account right before Chase decided to close it before they can release the final check to us of the remaining balance.
Just before the closure, I received all of my backed up unemployment and pandemic assistance benefits (over 12k) and now Chase wants me to prove that I was in fact entitled to that money. I sent in all documentation I had from DOL but Chase still refuses to release the money and says they are still ‘investigating’.
Since when do I need to justify to my bank whether I was entitled to unemployment benefits? How come they can hold my money for over 3 months now without ANY explanation or communication from their side? I’m furious and outraged by this whole situation, but we have no one we can reach. My branch can’t help, since all they can do is call an executive number when I go in to inquire and they are given the runaround as well.