At a news conference, Rutger Jeuken from the public prosecution service said authorities are considering that the attack had terrorist motives, and maybe others.
Meanwhile, the Dutch anti-terror coordinator has raised the threat alert to its highest level around the central Dutch town of Utrecht following the shooting.
Security services have reportedly told Utrecht's University Medical Centre to open the dedicated emergency ward to help care for the injured, reported BBC. The incident "currently has all the characteristics of a terrorist attack", NATC said.
The city was put into lockdown after the shooting, shortly after the morning rush hour, which authorities initially said was an apparent terrorist attack.
The shooter remained at large, prompting the Dutch government to put the country's military police on "extra alert" at airports and "vital buildings" amid the manhunt.
Dutch Prime Minister Mark Rutte said he is "deeply concerned" about the incident and all political parties have suspended their election campaigns.
Think your friends would be interested? There was no immediate confirmation on that from Dutch police.
Several injured in shooting on Netherlands tram
Police have urged members of the public to stay away from the area while investigations are ongoing and no trams are travelling through the city.
Gokmen Tanis was arrested by Dutch police for his involvement in a shooting in Utrecht on March 18.
Police spokesman Bernhard Jens said one person might have fled by vehicle and he did not rule out the possibility that more than one assailant was involved.
Police spokesman Joost Lanshage said: 'Several shots were fired in a tram and several people were injured. Police have noted that there is now a lot of uncertainty and they have no further information to provide to the public at this stage.
Utrecht, the Netherlands' fourth largest city, has a population of about 340,000.
Police erected a white tent over an area where a body appeared to be lying on the ground, but there was no official confirmation of fatalities. The gunman opened fire and fled, eyewitnesses said.
Swansea City 2 Manchester City 3: Controversial Aguero victor completes comeback
It was a dire situation that left City in danger of a repeat of last season's embarrassing FA Cup exit at Wigan. Championship side Millwall host Premier League side Brighton on Sunday.