Thanks for asking your question! I'm sorry to hear about your tax issue and I'm going to try my best to help you understand or resolve it.
I'm truly sorry to hear about your loss.
Form 706-NA is not the transfer certificate. The transfer certificate is IRS form 5173
"Transfer Certificate" (IRS Form 5173) is received from the Internal Revenue Service. Executors, surviving joint tenants, trust beneficiaries or other persons legally entitled to receive the decedent's assets can request and obtain the Transfer Certificate from the
IRS SB/SE Estate Tax Group 1205,XXXXX, N.E. (UCP-CNN-730), Washington, DCNNN-NN-NNNN U.S.A.; telephone:(NNN) NNN-NNNN fax:(NNN) NNN-NNNN
Customers will likely be asked to provide the IRS with copies of the estate tax return and
corroborating documentation. In addition, the IRS will typically require copies of the decedent's last will and testament and codicils thereto, if any, with English translations if appropriate, as well as copies of any death or inheritance tax returns and corrective
statements filed with a tax or regulatory authority outside the U.S., and a copy of the decedent's death certificate.
Use the IRS contact information supplied above so you can be assisted with the documentation required. The IRS will then be able to advise you what if anything you need to send them so Merrill can release the funds.
Form 706-NA is the estate tax return that you would need to fill out -you will need this in addition to the transfer certificate mentioned above.
Your mother will owe estate taxes on her US securities, and this is true whether or not she placed those securities in the US or not. I would definitely have a tax professional handle all this for you, as it is a very complex area of tax. I could not tell you how much she would owe, because to tell you that I would need to complete the forms for you.
You don't have to have an attorney - you can use a CPA or EA to do the same.
More information on this topic can be found, here: http://www.irs.gov/Individuals/International-Taxpayers/Some-Nonresidents-with-U.S.-Assets-Must-File-Estate-Tax-Returns
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