Dutch authorities said Tuesday they were "seriously" investigating a possible terrorist motive for the Utrecht tram attack because of evidence including a letter found in the gunman's getaway car.
Police were questioning Turkish-born main suspect Gokmen Tanis, 37, and two other men over Monday's rampage in which three people were killed and seven injured, three seriously.
Dutch Prime Minister Mark Rutte had earlier said they "cannot exclude" other motives including a family dispute, but police and prosecutors said on Tuesday that the probe was leaning towards terrorism.
"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," police and prosecutors said in a joint statement.
"Our investigation has established no link between the main suspect and the victims."
The three people who died in the shooting were a 19-year-old woman from Vianen, south of Utrecht, and two men aged 28 and 49 from Utrecht itself, the statement said.
Armed police captured Tanis after an eight-hour manhunt that virtually shut down the Netherlands' fourth largest city and triggered a nationwide increase in security at airports and key sites.
Police said they had found a red Renault Clio used as a getaway car after the attack. They had also found a firearm after his arrest, they added.
Tanis and two other men aged 23 and 27 are still being interrogated, police said.
A stream of mourners laid flowers on Tuesday at the site of the attack near the 24 Oktoberplein square.
"One of the victims was my friend's girlfriend. So coming here today was the least I could do," Marco van Rooijen, 43, told AFP.
"I am here to pay homage to the victims and to support their families," added local resident Yvette Koetjeloozekoot, 29.
Flags were flying half-mast on many buildings around the Netherlands.
Public transport was running again after forensic police finished their investigations at the scene and removed the tram on which the shooting erupted.
Rutte was chairing a cabinet meeting on the attack, which has raised security fears a day before provincial elections in the Netherlands.
Turkish President Recep Tayyip Erdogan said his country's intelligence agency was "looking into" the attack.
"Some say it is a family matter, some say it is a terror attack ... Our intelligence agency is looking into the matter", Erdogan told Ulke TV late Monday.
Dutch media have reported that the suspect had a long rap sheet and was only released from jail two weeks ago on a number of charges.
Broadcaster NOS reported that some people close to him had links to fundamentalist Islamic groups, but also that he had divorced his wife two years ago.
Police and witnesses said gunfire broke out on the tram on Monday morning, sending people fleeing and triggering a huge police response.
Mosques and schools were closed across Utrecht following the bloodshed, before heavily armed officers surrounded a building and arrested Tanis.
Support for the Netherlands poured in from around the world including the United States, the EU and Russia.
US Secretary of State Mike Pompeo said: "America stands with you. We will continue to do all we can to help you in this terrible time of tragedy."