My bank cashed a check on account that was closed for 5 months. Now I'm responsible for funds. Advice?

I was recently scammed. An x-friend business affiliate wrote a check for $10,000.00, deposited into my Chase account on 9/12. Check came from another Chase account. Next day, 9/13, I went to my local branch, asked teller if he could confirm the funds were in that account since it was Chase to Chase. I was then given cash in 100s. Then told the check was written from an account that had been closed for 5 MONTHS!!! X-friend vanished, of course to avoid a possible lynching. Chase's Fraud Investigation says even if teller was careless, the bank's system should have flagged this check. I'm then told that the Chase check came from what was formerly WAMU and they are still being converted to Chase therefore the system may not have picked up on it and it "slipped through the cracks." They are insisting that I pay the full negative balance of $8,000.00 to avoid CheckSystems and having my credit ruined. They have not agreed to absorb the cost because ultimately I took the money and am responsible. I've filed criminal charges with City's DA against this person. If he is not caught, I am still responsible for the money. Didn't Chase's mistake cause me $10K? Aren't they also to blame? Any advice? Mine is to be careful WITH ANY Chase to Chase or WAMU transactions for now. Can someone please tell if I have a chance? Should I Chase what matters?