After six raids in Brussels and elsewhere, Belgian police rounded up four people they suspect of involvement with the Thalys attack.
They found no explosives or arms.
Armed with a Kalashnikov assault rifle, a pistol and a box cutter, Moroccan national Ayoub el Khazzani opened fire on passengers on the Thalys train travelling from Amesterdam to the French capital.
But he was overpowered by two American tourists, Alek Skartakos and Spencer Stone, who disarmed him with the help of British citizen Chris Norman and US student Anthony Sadler.
Two of Monday’s raids were in the Brussels district of Molenbeek, where Khazzani grew up and where Paris attacker Saleh Abdeslam was arrested last year.
Khazzani has told investigators he was acting on orders from Abdelhamid Abaaoud, one of the organisers of the 2015 Paris attacks whom he met while fighting for the Islamic State armed group in Syria.