Marcel Henri Barbeault was born on August 10, 1941, in Liancourt, France. He was the oldest son in his family. His father worked as a steam locomotive conductor, while his mother was employed in the textile industry. Marcel did not stay in school for long. He quit at a young age after he failed to receive his primary education certificate.

At the age of 14, he began working at a workshop called "Établissements Rivière" in Creil as a riveter. In his youth, he joined the Young Christian Workers (YCW) organization. On December 13, 1960, Marcel joined the army. He was called to serve during the Algerian War and worked as a stretcher bearer.

After returning from military service, Marcel went back to work at the factory in Saint-Gobain. He became interested in boxing and judo. He hoped to become a paratrooper or a gendarme, which is a type of police officer. However, he suffered from vertigo, which caused him to fail the tests for these positions multiple times.

In 1964, Marcel married a woman named Josiane. Together, they had two sons: Patrice, born in 1966, and Laurent, born in 1972. In 1968, his life changed dramatically when his mother, Micheline, passed away from cancer. This loss was followed by the deaths of his two brothers.

On January 10, 1969, Marcel Barbeault committed his first murder by killing Françoise Lecron, the wife of a Saint-Gobain engineer. His last murder was in January 1976. Barbeault targeted his victims along a railway line after dark, usually between 9 PM and 11 PM. He used different methods, including hitting them with a shovel and stabbing them in the heart, or shooting them with a .22 Long Rifle. His victims were rumored to be women of color, and he watched them for a long time before attacking. After killing, he would undress the bodies but did not sexually assault them. He typically stole their handbags.

Despite his crimes, Barbeault was seen as a good husband and father. For years, he managed to avoid police capture. He was able to evade suspicion because of his normal demeanor. After seven years of hunting, he was arrested after receiving an anonymous tip that described him. He was identified as a suspect by Commissioner Christian Jacob. Inspector Daniel Neveu connected Barbeault to the murders through a rifle found in his basement and a bullet discovered in a cemetery.

Neveu focused on the cemetery in Nogent-sur-Oise, as it was central to where the murders occurred. Additionally, a double homicide in a cemetery parking lot was attributed to him. The police noted that the days of the murders matched his days off from work. When investigators searched Barbeault's home, they found a sawed-off rifle, a raincoat, and caps. Ballistic tests linked the rifle to one of the murders.

Barbeault’s trial began on May 25, 1981. He faced charges for five murders, but three other charges were dropped due to lack of evidence. Despite clear evidence, Barbeault denied being "The Shadow Killer." He was cold during the trial. Initially, there was an attempt to seek the death penalty, but this did not proceed due to recent changes in political leadership in France, which led to the abolition of the death penalty.

On June 10, 1981, Barbeault was found guilty and sentenced to life imprisonment. He attempted to appeal the sentence, and although his appeal was initially successful, the original sentence was later reinstated in November 1983. As of now, Marcel Barbeault remains in prison and works as a librarian.