Dutch prosecutors investigating the murder of three people on a tram said a letter found in the gunman's getaway car was among the reasons why a terrorist motive was being seriously considered.
Three people were killed and three others seriously wounded in the attack in the central city of Utrecht.
Turkish-born suspect Gokmen Tanis was arrested on Monday evening.
No connection has been found between Mr Tanis and the victims of the attack.
Police said on Tuesday that armed officers had arrested another suspect, a 40-year-old man, in Utrecht. A statement said he was "suspected of involvement" and his role was under investigation.
Two other men arrested on Monday were released and were no longer suspects, police said.
Prime Minister Mark Rutte laid flowers at the scene of the attack on Tuesday afternoon. In a statement he paid tribute to the people of Utrecht who had "remained calm despite the great threat".
What did prosecutors say?
"So far a terrorist motive is being seriously taken into account. Among other things a letter found in the getaway car and the nature of the facts give rise to that," a statement said (in Dutch), without detailing the contents of the letter.
Prosecutors say a firearm was found when Tanis was detained. They say other motives have not been ruled out.
The red Renault Clio getaway car was stolen from the scene of the attack and later found in Utrecht's Tichelaarslaan close to where the suspect was arrested.
Owner Frans Hoogendijk said he had abandoned the car with the engine running when he saw people fleeing the tram and one shouted at him to run for his life.
"I thought it was a bomb and leapt out of my car," he told the AD website.
Neighbours had earlier described the suspected gunman as a "loser" and a petty criminal rather than a terrorist.
Dutch justice officials confirmed on Tuesday that he had been released from custody recently in a rape case, which was due to go to court in July. He was freed after promising to co-operate with authorities. He has been convicted of crimes this month as well as in the past.
According to Dutch law Tanis must appear in court by Thursday.
Who are the victims?
Two of the three people who died have been named:
- Roos Verschuur, 19, worked in a snack-bar in Vianen south of Utrecht
- Rinke Terpstra, 49, was a football coach with the Desto youth club - he had three children
- The third victim was a 28-year-old man from Utrecht
Three of the wounded are in a serious condition and Mayor Jan van Zanen visited one of them in hospital on Tuesday. Two are women in their early 20s and a man in his 70s.
Flags were flying at half-mast on public buildings in the Netherlands on Tuesday and flowers were laid at the site of the attack in Utrecht's 24 Oktoberplein.
Three people were killed by the shooting. The victims are a 19 y/o woman from Vianen and two men from Utrecht. They were 28 and 49 y/o. Also three people were severely injured (a woman (20) from Utrecht, a man (74) from De Meern and a woman (21) from Nieuwegein. #24oktoberplein— Politie Utrecht (@PolitieUtrecht) March 19, 2019
At the scene
Anna Holligan, BBC News, Utrecht
Sarah cycled to the scene to lay a fresh bouquet of tulips, her 13-month-old toddler, Jet, harnessed in a seat on the front of the bike.
"I felt afraid to come out," she told me. "But I had to show my daughter that we have to be strong, to keep living."
A note pinned to a bunch of roses read, "It hit right through the heart."
A Muslim man fell to his knees, his head bowed and hands cupped in silent prayer.
The victims appear to have been targeted at random. Among the tears and shows of solidarity, the overwhelming sense among the people is, it could have been one of us.
What is known about the suspect?
Police released Gokmen Tanis's name after the shooting as they searched his home and other addresses.
Prosecutors said he was known to police while friends told Dutch media that he had been in trouble on a number of occasions.
Turkey's President Recep Tayyip Erdogan said his country's intelligence service was "looking into" the attack.
Gokmen Tanis was not known for extreme religious beliefs, although he was once captured on video criticising a female reporter's clothing.
A 47-year-old woman called Angelique told Algemeen Dagblad that Tanis was a drug user with a criminal record who was facing trial for raping her in 2017. She said he had recently been freed from custody after violating the conditions of an earlier release.
"He's not a terrorist but a psychopath," she said.
The case is due to return to court during the summer.
Earlier this month he was given jail terms for shoplifting as well as burglary. In 2014 he was cleared of attempted manslaughter but given a jail term for illegal arms possession.
Several reports said his marriage had fallen apart. A neighbour who grew up with the suspect told De Volkskrant newspaper that he did not go the mosque and the case had nothing to do with religion. "He's a lost boy with the IQ of a shrimp."
What happened on Monday?
At about 10:45 local time (09:45 GMT), police were called to reports of a shooting on board one of the city's trams at the 24 Oktoberplein junction.
One witness told local media that "a man started shooting wildly".
Another witness told Dutch public broadcaster NOS that he had helped an injured woman when the tram came to an emergency stop.
"I looked behind me and saw someone lying there behind the tram," he said. "People got out of their cars... and they started to lift her up.
"I helped to pull her out and then I saw a gunman run towards us, with his gun raised," he said. "I heard people yell 'Shooter! Shooter!' and I started to run."
The gunman then fled the scene, leading to the manhunt which lasted for much of Monday.
A number of raids were reportedly carried out and counter-terrorism officers were pictured patrolling the streets near to where the attack happened. Police surrounded a building not far from the scene of the attack and arrested Mr Tanis on Monday evening.