It shouldn't be that difficult. Accounting for money has been around since people started trading shells & rocks. But of course it would be easier if they kept track of it before it was spent. I don't know how this was run, but most gov't contractors &/or large projects have government accountants & auditors on site to track costs & expenditures during the project.
Again, I don't know how they did it, but first of all the projects w a budget should have been approved before they even began. Project A costs $XXX, Project B costs $XXX...Then the money should be allocated to each phase of each project - not just "Here's $16 billion, go have fun"
Then on an ongoing monthly, or at least quarterly basis, actual cost reports are run & compared to budgets. Every deviation over a certain dollar amount or percentage, needs to be explained by a project manager, and overall project deviations need to be explained by the chief project manager.
Yes, expenditures should be able to be traced to a Request for Purchase, Purchase Order, Receipt of Purchase/Packing Slip, Invoice, and Disbursement of Funds. Also Payroll Reports, Time Cards, Paychecks. No penny should be unaccounted for.
These controls should have been put in place before & should have been ongoing throughout the project. It would be more difficult now, but audits can & should be done tracing expenditures to bank accounts & the above documents. A physical audit of assets/inventories should be done as well as audits of the personal bank accounts of any person suspected of fraud.
It should be very easy to see what happened with this money. Cheques have to be written. Payments made by any means have a trail. For all money not there that should be, it’s MISSING. Missing money is stolen money. Nothing happened to Hillary when $6B was MISSING from State Dept. Hopefully that is being investigated along with everything else.
Of course, but we are talking big city Democrooks - https://www.redstate.com/slee/2019/03/05/bill-de-blasios-wife-case-missing-850-million-taxpayer-money/