The man, identified by French media as Francois Verove, was found dead on Wednesday in the southern town of Le Grau-du-Roi, on France’s Mediterranean coast.
Verove’s DNA has been matched to evidence found at several crime scenes, prosecutors said. Further tests are being carried out.
Le Grêlé is blamed for four murders and a series of rapes since 1986 in cases that, until now, have remained unsolved.
The crimes include the 1986 murder of an 11-year-old child, Cécile Bloch, in Paris.
Le Grêlé’s other suspected murder victims included one man, Gilles Politi, 38, a 20-year-old German woman, Irmgard Müller, and 19-year-old Karine Leroy.
In his note, Verove confessed to committing murders without providing details of the victims or circumstances, French media reported.
He reportedly admitted to experiencing “previous impulses” before he “got himself together”.
The contents of the letter have not been confirmed by French authorities.
French media said Verone, who was reportedly retired, took his own life after failing to comply with recent requests from authorities investigating the Le Grêlé crimes.
He is said to have had two children.