Speaking in an interview with the daily De Tijd Mr Vandoren adds that from the outset his service did take account of the possibility that the perpetrator was a Syria-fighter.
"We must await the outcome of the investigation, but I feel this is not the work of a terrorist acting alone."
Mr Vandoren says information about the suspect points in this direction: "He committed the outrage alone, but he underwent training and was radicalised. The investigation must now map out exactly what happened: How was he trained? How could he remain out of sight in the days after the attack?"
The daily De Tijd believes some 70 Syria-fighters have returned to Belgium and that at least 50 of this number require an extensive follow-up. Mr Vandoren confirmed that several dozen people were involved.
He believes most returning Syria-fighters are dangerous, but urges vigilance with regard to all forms of terrorism linked to Al-Qaeda.
"It is clear that Belgium was one of the first nations to assess the threat from Syria-fighters clearly. We are trying to gain an accurate picture, but it's no walk in the park because it remains very difficult checking matters out in Syria."