Four people have been detained in Belgium in connection with a terrorism investigation into an attack two years ago on a Thalys express train to France that was foiled by three Americans.

The Federal Prosecutor's Office said Monday that six houses in the Brussels area and in southern Belgium have been searched in the operation. An investigating judge was to decide later whether those taken in for questioning should remain in custody.

Authorities said no arms or explosives were found in the searches. In August 2015, a man on a Thalys train that had just crossed into France from Belgium tried to open fire with an assault rifle but was overpowered by three Americans, two of them off-duty U.S. military members.

French police termed it an Islamic extremist attack.