According to Le Monde, the prosecutor's office said that a suspect had been taken into custody that same evening and that the man, who was born in 1985, has been known to authorities "for similar acts" in the past, said Saint-Omer's prosecutor Mehdi Benbouzi, who spoke to Agence France-Presse.
Government ministers including Culture Minister Rachida Dati expressed "solidarity" with the residents of Saint-Omer.
The parish priest, Father Sébastien Roussel, entered the burning church to evacuate the consecrated hosts. "I wanted to evacuate the Blessed Sacrament," he reported after successfully doing so and coming out unharmed.
Bishop Olivier Leborgne of Arras, Boulogne and Saint-Omer said in a Sept. 2 statement that "It was with shock that I learned early this morning that the Church of the Immaculate Conception in Saint-Omer was suffering a particularly violent and destructive fire."
120 firefighters contained the flames and no one was hurt, although more than 50 residents living nearby were evacuated as a precaution, officials said.
"I want to tell all those who attended this church, regularly or for the big moments of life, of my closeness in prayer," he wrote.
On Sept. 4 he met the local community and celebrated a morning Mass in the town's Basilica of Our Lady of Miracles.
"I entrust you all -- residents of Saint-Omer, firefighters and professionals working to ensure the safety of people and places -- to the Virgin Mary, to whom this church is dedicated," he said of the Church of the Immaculate Conception, which originated in the mid-19th century and was only restored and reopened in 2018.
Interior Minister Gérald Darmanin said on X, formerly Twitter, on Sept. 2, "My thoughts are with the Catholics and the people of Saint-Omer,"adding that "an investigation is under way to determine the exact cause of the fire."
An advocacy group based in Vienna, Observatory on Intolerance and Discrimination Against Christians in Europe, said the alleged perpetrator "was known for similar activities. He has attempted to set fire to 15 churches and has been convicted 25 times." The group added that he followed "Islamic content."
The town of Saint-Omer has a historical connection to the Catholic Church in the United States and its first bishop -- John Carroll of Maryland. According to charlescarrollhouse.org, when John turned age 13, his family sent him and his cousin Charles to study with the Jesuits at the College of St. Omer in France's Artois region.
Ordained a priest on Feb. 14, 1761, Father Carroll in 1789 became the first bishop of the new United States, leading the new Diocese of Baltimore, which then included all 13 original states. His cousin Charles was the only Catholic signer of the U.S. Declaration of Independence.