Auguste Montferrand died on June 28, 1858, a month after the completion of the cathedral and 40 years after the start of its construction. The memoirs cite very different reasons. The newspaper "Severnaya Pchela," reporting on the funeral service at the French Church of St. Catherine, wrote that his home was a house of education, friendliness, and warm hospitality, where great artists, connoisseurs of the fine arts, famous people, and modest artists gathered. The famous writer Alexandre Dumas was present in St. Petersburg at that time. In October of the same year, the French writer Théophile Gautier also visited St. Petersburg, and the new St. Isaac's Cathedral made such an indelible impression on him that he devoted 30 pages to it in his book.

"S.-Peterburgskie Vedomosti" published an official announcement of the death of the architect of St. Isaac's Cathedral, Actual State Councillor Augustin Augustinovich de Montferrand, in his 74th year, along with a brief obituary: "At Montferrand’s funeral, we learned of Ivanov’s death and could not help but compare... Montferrand was an artist who spent thirty (actually forty) years on his work; but with this work, he said his last word: he died having accomplished all he could, having enjoyed all that an artist can enjoy—he died like a commander after victory, amid cheers of triumph and honors...".

The mysterious death of the architect, moreover a foretold death, gave rise to the most fantastic guesses and rumors, spawning several versions of the tragedy. There were many versions, including mystical ones, but one of them is quite curious. The fact is that 19th-century Petersburg was a military capital, and at balls and on the streets, uniforms were seen more often than tailcoats or frock coats. At that time, the saying came into fashion: "Moscow is famous for its kalaches, Petersburg for its mustaches," because the distinctive privilege of the military, compared to civilians, was wearing mustaches, and needless to say, they took pride in their mustaches. Civilians were forbidden by law and etiquette to wear mustaches. According to this version, Alexander II at the consecration ceremony of St. Isaac's Cathedral suddenly noticed that Montferrand was wearing a mustache, although he was not a military man, and moreover a Frenchman, and the emperor publicly made a rather sharp remark to him.

Allegedly, feeling insulted, Montferrand left home before the ceremony ended, fell into depression, and soon died.

According to another legend, during the solemn consecration of the cathedral, one of Alexander II’s close associates drew the tsar’s attention to the sculptural group of saints on the temple’s pediment. Among them was a sculpture of Montferrand himself holding a model of the cathedral in his hands.

Depicting himself in such company was quite a bold move in itself, but that was not all: all the saints humbly bowed their heads before Isaac of Dalmatia, and only the architect remained with his head proudly raised. Another legend says that Alexander II noticed among the sculptures of saints, bowing to greet Isaac of Dalmatia, Montferrand himself holding his head upright. Noting the architect’s pride to himself, the emperor allegedly did not shake his hand or thank him for his work, which mortally upset Montferrand. Whether this is true is unknown, but the emperor did not grant permission to bury Montferrand in the cathedral (as the architect himself had wished). The most logical explanation is that August Augustovich was a Catholic. The architect was deeply upset, fell ill from distress, and died. This version is beautiful, but the cause of death was a quite ordinary attack of rheumatism following pneumonia. This happened exactly a month after the consecration of St. Isaac's Cathedral, which gave the townspeople reason to gossip that "the prophecy had come true."

The funeral service for the great architect took place on July 3 (15), at the Catholic Church of St. Catherine on Nevsky Prospect. Before that, the coffin with the deceased’s body was carried around his beloved creation—the St. Isaac's Cathedral—and a requiem was held inside the cathedral.

At Montferrand’s funeral at St. Catherine’s Church, the novelist Alexandre Dumas père was present. Shortly before, he had arrived in St. Petersburg at the invitation of Count Kushelev-Bezborodko. At that time, moved by the sad event, Dumas wrote a short article published in the "Journal de S. Petersbourg":

"Ghiberti, who was entrusted with sculpting the doors of the Baptistery in Florence, bent over them at the age of 20 as a young dark-haired man and straightened only as a 60-year-old old man. Montferrand spent the same time on his work, 40 years, almost half a century, more than the usual human lifespan, the time France needed to establish and overthrow three regimes (empires). But during these forty years, Montferrand not only created the doors of such a baptistery, he built an entire church, raised it, made it rise from the ground, made it soar to the sky. He not only sculpted bronze, he carved granite, polished marble, melted gold, set precious stones... While these two nations were at war, the alliance of art endured. With the compass of its architects, the pencil of its artists, France extended a hand to Russia...".

The main responsibilities for the architect’s burial were taken on by architect Auguste Poiroux. For 15 years, from 1843 to 1858, he was Montferrand’s right hand, and after his death, he became the chief architect of St. Isaac's Cathedral. Montferrand wanted to be buried in St. Isaac's Cathedral, but this was denied even during his lifetime.

After Montferrand’s death, the St. Petersburg magistrate announced in the press that Louis Léger Auguste Ricard de Montferrand, being a Court Councillor (later Actual State Councillor), made a spiritual will on September 22, 1835, by which he bequeathed all his movable and immovable property to his fiancée Élise Pic de Bonnier. Due to the loss of the original will, she presented a copy issued to her by the 2nd Department of the St. Petersburg Chamber of Civil Court, a detailed purchase deed for a stone house on the Moika River (2nd Admiralty part of the 2nd quarter, now the 4th quarter, under the number formerly 172, then 77), documents on the annexation of the wife's plot, and the metric certificate of Montferrand’s death. "She declared the value of the remaining property of her husband honestly: movable property 25,000 rubles, and the house 40,000 rubles, which according to the city table of 1846 was valued at 34,315 rubles 20 kopecks in silver." The department decided to recognize the will as valid and to introduce the widow into possession, collecting a fee from her.

At the widow’s request, the deceased’s body was placed in a metal coffin, sealed with wax, and sent to Paris, where the burial at Montmartre Cemetery was handled by Auguste Poiroux’s aunt. Many years later, Montferrand’s name was forgotten in his homeland, as was the location of his grave.

The architect’s grave was found by the French art historian and architectural historian Boris Nikolaevich Lossky (1906–2001).

He was a leading specialist in Franco-Russian cultural relations of the 18th–19th centuries. Based on archival information provided to him and after examining a number of cemeteries, he found Montferrand’s burial place. The search was complicated by the fact that in Paris, unlike, for example, St. Petersburg, there is no centralized collection of cemetery materials, and burial documents are zealously preserved by the caretakers of each individual cemetery, and information about the deceased is not always accessible.

In the early 1830s, Montferrand traveled to France specifically to create a monument for it. The tombstone is made in the form of a stone vault with a column installed on it, slanted at the top. The monument’s design was developed by Montferrand himself.

Montferrand was buried next to his mother—Marie Françoise Louise Fistioni—and stepfather, Antoine de Commarieux. She was the daughter of a merchant of Italian origin, Nicolo (Nicolas Joseph) Fistioni, and a Frenchwoman, Marie Françoise Hayez. She was born on November 9, 1768, and died on May 20, 1833. Below, unrelated to the previous text, the date of Montferrand’s death according to the Gregorian calendar is engraved: "July 10, 1858." On the front side of the tombstone, on a marble pentagonal slab, Montferrand’s monogram "AM" cast in bronze is attached.

The grave is located in the eastern part of the cemetery, not far from the main entrance, in section 18, which is listed on the cemetery street Chemin des Gardes. There lies a great man who found no place in the cemeteries of St. Petersburg.

Sources:

https://www.spb.kp.ru/daily/27230.5/4356468/

E. I. Krasnova: Unknown and little-known materials about Montferrand’s life (Montferrand without architecture)

Olga Alexandrovna Chekanova, Alexander Lukich Rotach: Auguste Montferrand

https://pantv.livejournal.com/659976.html

https://ru.wikipedia.org/wiki/Монферран,_Огюст