Dutch prosecutors yesterday said they were investigating a possible terrorist motive behind the shooting on a tram in the city of Utrecht in which three people were killed and five wounded.
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, who has a history of run-ins with law enforcement, was acting on political beliefs or a personal vendetta. "Other motives are not being ruled out," the statement said.
Under Dutch law Tanis must be brought before a judge by Thursday but does not yet have to be charged.
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.
Mahmut Tanis, an uncle of Tanis who lives in the Netherlands, told Turkey's state-run Anadolu news agency that he doubted radical motives. Tanis had previously been arrested, prosecutors said, but have declined to provide details.
There was no immediate comment from Tanis or any lawyer representing him.
Dutch Prime Minister Mark Rutte convened crisis talks immediately after the incident, which came three days after a lone gunman killed 50 people in mass shootings at two mosques in the city of Christchurch, New Zealand.
Utrecht went into lockdown in the aftermath of Monday's shooting on a tram just after the morning rush hour.
London, Paris and several other European cities have suffered militant attacks over the past years which have killed hundreds of people and authorities are on high alert.
Flags flew at half mast on government buildings across the Netherlands yesterday in tribute to the victims.