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.
Despite the terror suspicions, prosecutors said that "other motives are not being ruled out". Earlier, Mark Rutte, the Dutch Prime Minister, had said that the shooting may be linked to a family dispute.
Armed police captured Tanis after an eight-hour manhunt that brought the Netherlands' fourth largest city to a standstill amid a security lockdown.
They found a red Renault Clio that had he had used as a getaway car, and a firearm.
Reports said the two other suspects, aged 23 and 27, were brothers but unrelated to Tanis.
It emerged that Tanis, a repeat offender, had only been freed from jail in a rape case two weeks ago. He was also convicted in recent months for shoplifting and burglary.
Tanis was originally arrested in 2017 then released from pre-trial detention, before being taken back into custody when he breached his bail conditions, the central Netherlands district court said. He was freed again at the start of March.
In 2014, he was also convicted of "illegal possession of weapons" and attempted theft but acquitted of attempted manslaughter. While his relatives had links to fundamentalist Islamic groups, Tanis was reportedly known for unstable behaviour after divorcing his wife two years ago.