Dutch prosecutors investigating the murder of three people on a tram say a letter was found in the gunman's getaway car and it is one of the reasons why a terrorist motive is 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 detained on Monday evening.
No connection has been found between him and the victims.
Prosecutors say a firearm was found when Tanis was detained.
"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.
Other motives have not been ruled out.
This Renault car was found by police in Utrecht and is believed to be the getaway vehicle
The red Renault Clio was stolen from the scene of the attack and later found in Utrecht's Tichelaarslaan close to where the suspect was arrested.
Neighbours had earlier described the suspect 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.
Two other suspects are in custody, aged 23 and 27, and authorities are assessing if they had any involvement in the attack. Their lawyer told Dutch media the men were brothers but not related to the main suspect.
Two of the three people who died have been named:
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 twenties and a man in his seventies.
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.
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.
Teenagers were among many laying flowers at the scene of Monday's shooting in Utrecht on Tuesday
"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.
Prime Minister Mark Rutte took part in a minute's silence in the Dutch parliament on Tuesday, in honour of the victims of the Utrecht shooting and the Christchurch mosque murders last Friday.
The message "Utrecht, stay strong" has appeared on billboards in the city.
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 said the intelligence service there was also looking into the attack.
The threat level was temporarily raised to its highest point in the province of Utrecht
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."
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.
Police surrounded a house earlier on Monday