Abstract: The sudden death on December 3, 1845 of the famous public figure, historian and writer Alexander Ivanovich Turgenev caused a lot of sympathetic responses. Many «mournful» letters were received by an old friend of the deceased, the poet V. A. Zhukovsky. This article attempts to prove for the first time that the author of one anonymous letter sent to Zhukovsky was Turgenev's close friend Ekaterina Aleksandrovna Sverbeeva. This assumption is justified by comparing «the letter of an unknown person about the death of A. I. Turgenev» with other letters of A. I. Turgenev's friends and acquaintances (both published and introduced into scientific circulation for the first time). The first part of the article describes the relationship of A. I. Turgenev with the Sverbeev family; the second part compares «the letter of an unknown person» with the letters of N. A. Melgunov to V. A. Zhukovsky; the third part compares «the letter of an unknown person» and one of the messages of E. A. Sverbeeva to A. N. Popov. As a result of the conducted research, it can be stated with full confidence that the author of «the letter of an unknown person» was E. A. Sverbeeva. Firstly, it is clear from the usage of words in the compared letters that the addressee was in close friendly relations with A. I. Turgenev (the case of E. A. Sverbeeva); secondly, «the letter of an unknown person», the letters of N. A. Melgunov and E. A. Sverbeeva have several similar plots and textual coincidences. The materials used in the article may be useful in the further development of the topic of A. I. Turgenev's relations with the Sverbeev family, as well as expand and detail the idea of the intellectual atmosphere in Moscow of the Nicholas time. The article also touches upon the general problems of the methodology of source studies: questions about possible ways to find the author of the document and the limits of using the comparative method in the analysis of epistolary sources.