Flowers have been set up in tribute to victims at the site of a shooting in a tram in Utrecht.
A Turkish-born man, 37-year-old Gokmen Tanis, was arrested after a seven-hour manhunt on Monday by security forces and remained in custody.
Prosecutors said he is suspected of three fatal shootings, possibly with terrorist intent. Two other suspects were also in custody, police said, but their role was unclear.
"Up to this point, a terrorist motive is seriously being considered," prosecutors said in a statement, citing "the nature of the shooting and a letter found in the getaway car."
But it remained unclear whether Tanis was acting on political beliefs or a personal vendetta. "Other motives are not being ruled out," the statement said.
The three Dutch victims were identified as a 19-year-old woman and two men aged 28 and 49. Three others, ranging from 20 to 74 years old, were critically injured in the shooting.
Prosecutors said they had so far not been able to establish a connection between the victims and the suspected gunman.
Tanis had previously been arrested, prosecutors said, but have declined to provide details.
Flags flew at half mast on government buildings across the Netherlands on Tuesday in tribute to the victims.