Dutch prosecutors said they were "seriously" investigating a terrorist motive for the Utrecht tram attack after discovering a letter in the gunman's getaway car.
Police were questioning the prime suspect, Turkish-born Gokmen Tanis, 37, and two other men over the shooting in which three people were killed and seven injured, three of them seriously.
"So far, a terrorist motive is seriously being taken into account. This is based on a letter found in the getaway car among other things and the nature of the facts," said prosecutors and police in a statement.
"Our investigation has established no link between the main suspect and the victims."
The three people who died were Roos Verschuur, a 19-year-old woman from Vianen, south of Utrecht; Rinke Terpstra, 49, football coach with the Desto youth club and father of three; and a third, unnamed 28-year-old man from Utrecht.