Michael Goolaerts, who died on Sunday, suffered cardiac arrest while riding, causing his crash at Paris-Roubaix.
The Veranda's Willems-Crelan rider, 23, was found unconscious and not breathing after falling on a cobbled section of the one-day classic.
The Belgian was airlifted to hospital in Lille but died at 21:40 BST.
"He suffered an attack while racing. His heart stopped, and that's why he crashed," said Cambrai state prosecutor Remi Schwartz, following an autopsy.
Schwartz said experts will carry out toxicology and pathological exams to determine the cause of the cardiac arrest, although the results may not be known for several weeks.
Goolaerts crashed on the second of 29 cobbled sections in Paris-Roubaix, 109km into a 257km race regarded as one of the toughest and most prestigious in cycling.
Goolaerts' team return to racing
Veranda's Willems-Crelan rode their first race since Goolaerts' death at the one-day Brabantse Pijl in Belgium on Wednesday.
The Belgian outfit decided to compete after consulting with Goolaerts' family and their riders.
Veranda's Willems-Crelan led the peloton in a minute's silence at the start in Leuven and riders wore black armbands.
Belgian Tim Wellens, who rides for Goolaerts' former team Lotto-Soudal won the 201.9km cobbled race, pointing to the sky as he crossed the line.
Dutchman Huub Duijn was the highest-placed Veranda's Willems-Crelan rider, finishing eighth.