PARIS, Aug 9 (Reuters) - A Rwandan immigrant in France already under investigation for setting the Nantes cathedral on fire last year has been arrested on suspicion of murdering a Roman Catholic priest, prosecutors said on Monday. The 40-year-old man surrendered to police mid-morning and admitted to killing the head of the religious congregation where he was staying as he awaited trial for the cathedral fire. La Roche-sur-Yon prosecutor Yannick Le Goater told reporters police had found the body of 60-year-old Olivier Maire, head of the Montfortain Missionary Order at Saint-Laurent-sur-Sevre, in a room to which the suspect had given them the key. "At this stage of the investigation there seems to be no terrorist motive whatsoever," Le Goater said. Following the cathedral fire in July 2020, the suspect was held in detention until late May when he was then released under judicial supervision and placed in the abbey, the prosecutor said. After talking about wanting to leave, he was transferred to a psychiatric hospital where he stayed until the end of July. The killing will raise pressure on President Emmanuel Macron over his security and immigration policy, eight months out from a presidential election that will see his biggest challenge come from the far right. "So in France you can be an illegal immigrant, torch Nantes cathedral, never be expelled and reoffend by murdering a priest. What"s happening in our country is unprecedented: the total failure of the state," tweeted far-right leader Marine Le Pen. France"s immigration authorities had handed the Rwandan national an expulsion order in 2019, but he remained in the country as he awaited trial. Prosecutors allege that a year later he set fire to Nantes cathedral. The July 2020 blaze engulfed the inside of the 15th-century cathedral, destroying a grand organ, stained-glass windows and a painting. Local worshippers described a priest much loved by his parishioners and known for his profound homilies. "I"m in shock. I cannot believe it to be," Sister Dorothee Harushinana, who attended a Mass the priest had led on Sunday, told Reuters by telephone. "He was someone close to the people. You could always call on him."