Goolaerts was unconscious and not breathing when medics arrived on the scene of his fall. They carried out rescusitation efforts until he could be airlifted to a hospital in Lille, where he was unable to be revived.
The Veranda's Willems-Crelan rider, 23, was pronounced dead on Sunday evening.
"The autopsy confirms the previous hypothesis that death was due to a heart attack and not a crash," said Remy Schwartz from the Cambrai prosecutor's office, according to AFP. "He suffered an attack while racing. His heart stopped, and that's why he crashed."
The authorities are carrying out a non-criminal investigation into the young rider's sudden death, which will continue with further tests.
"For now, we will carry out toxicology and pathological exams to determine the real cause of the attack," Schwartz said.
The news comes as Goolaerts' Veranda's Willems squad honoured his memory at the start of De Brabantse Pijl in Overijse, Belgium. The team competed in the race, with Dries De Bondt making the early breakaway. Organisers of the race held a minute of silence in honour of Goolaerts.
Veranda's Willems replied with a post on Twitter that read, "A very emotional day for all of us, but boy are we proud of our riders today, those who were riding and those who were there to help! Thanks to all of you for your massive support! #ALL4GOOLIE ”