The 40-year-old German, who has worked as a youth leader and had already been questioned in the case four years ago, admitted to killing a nine-year-old boy called Dennis in 2001, as well as two other boys aged eight and 11 in the nine years before.
He also confessed to sexually abusing "several" minors.
After he was arrested on Wednesday the unnamed serial murder suspect told investigators in the northern city of Hamburg that he drove one of his victims to Denmark and buried his body in a sand dune on a beach.
Authorities in Verden, northern Germany told reporters he may have also killed an 11-year-old French boy, Jonathan Coulom, who disappeared from a camp in western France in April 2004 and whose body was recovered six weeks later.
He is also a prime suspect in the death of another boy, 11-year-old Nicky Verstappen, in 1998 in the Netherlands, although he has denied involvement in either case.
"We want to determine whether we are aware of all of this man's deeds or whether he may have committed other crimes," the head of the regional police investigation unit spearheading the case, Karsten Lemke, said.
The unit was established to hunt the killer of Dennis, who vanished during a school field trip on September 5, 2001.
Mushroom pickers discovered his body about 40 kilometres away two weeks later, but the killer's trail had long gone cold.
But earlier this year, a witness who saw a television programme about the case recalled seeing a station wagon parked on a forest path at about 4:30 am in early September 2001 with a boy who resembled Dennis in the back seat and a man in the front, investigators said.
The witness, who was a soldier at the time, was training for a marathon before he had to report for duty that day.
The brawny, bespectacled man in the car, who looked to be in his early 30s, matched a description police had received of the suspect from sex abuse victims.
Between 1992 and 2004, five boys were killed in the same way in Germany, France and the Netherlands while at least 40 children were sexually molested in the same regions during that period, according to media reports.
Police had long suspected the murderer to be a serial killer living in Germany.