"Letters of Office" is not really a letter. If decedent died with a Will, a Petition to admit the Will to probate must also be filed along with a Petition for Letters of Office.Working your way down, you'll fill out the name and address, as well as phone number. Who exactly is this "to? Edu or call 773.702. A letter of testamentary gives an estate's representative the legal authority to administer the deceased's estate. The Petition for Letters of Administration is the document that is used when the decedent died without a will. First you will need to obtain a copy of the death certificate to present to the court. The powers you give your agent are explained more fully in Section 34 of the Illinois Power of Attorney Act. This form is a part of that law.