My 1040 Line 64 is reporting $19,000 even after I have deleted all income (W-2s, 1099-Rs, etc.). How do I find out where this is coming from so I can delete it ?

Its easy with the desktop software, you switch to "Forms Mode", and look a the "Tax Payments Worksheet"  which breaks down all withholding by form type.

To do that with Online, you need to pay for your software with a credit card......go to the Print Center on the left side of the screen, and save you tax return to a PDF...with ALL Forms and Calculation Worksheets....and then look for the "Tax Payments Worksheet" in that set .  IT should prompt you to pay at that time (if you haven't already.)

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Other forms that might have Federal withholding on them include.

SSA-1099, RRB-1099, RRB-1099-R

W-2G

1099-MISC

1099-G for unemployment (not state refunds)

1099-K

Schedules K-1

Occasionally, 1099-DIV, -INT or -OID

.....and brokerage forms 1099-B  (could be real, but usually it's people who  enter their ISO, or NQSO employee stock options exercises there, but accidentally include their Federal withholding when the withholding was actually already on their W-2 form itself)