Dutch tram attack letter, 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.