Can you determine who unreconciled an account that had already been reconciled?

Yes, you can determine who unreconciled the account by running the Audit Trail report, @DTT2. Happy to outline the complete steps to achieve your tasks. 

It's important to know what our users do in our company file. You can run the Audit Trail report and customize it to show the specific activities. Proceed with the following steps:

1. Go to the Report menu and select Accountant & Taxes. 
2. Pick the Audit Trail tab. 
3. Click the Customize Report button. 
4. Choose the Filters button. 
5. Type account in the search box and choose specific bank accounts. 
6. Type Date in the field and filter the appropriate period. 
7. Hit OK. 

For more details about customization, check out this article: Customize reports in QuickBooks Desktop.

Additionally, check out these articles below on how to combine reports from two or more company data files, customized or memorized reports, and other reconciliation topics: 

• Understand reports in QuickBooks
• Fix issues when you're reconciling in QuickBooks Desktop

If you have further questions about reports or reconciliation, please do not hesitate to let me know in the comments section. I'd be delighted to assist you and resolve the situation as soon as possible. Have a wonderful day, and take care!