Three people were killed and nine others injured in a shooting in a tram in the central Dutch city of Utrecht, the mayor said, Dutch broadcaster NOS reported.
Police said they were looking for a 37-year-old man of Turkish origin identified as Gokmen Tanis.
"In Utrecht there was a shooting at several locations," Netherlands Counter-Terrorism Agency head Pieter-Jaap Aalbersberg told a news conference. "A lot is still unclear at this point and local authorities are working hard to establish all the facts. What we already know is that a culprit is at large.
He declined to comment on the number of injured or what condition the victims were in.
Dutch Prime Minister Mark Rutte said all efforts were focused on catching the "suspect or suspects" responsible for a shooting in Utrecht that is believed to have killed one person.
Rutte did not repeat earlier comments by counter-terrorism authorities who suggested the shooting may have a terrorist motive.
"Our country today has been jolted by an attack in Utrecht," Rutte said in a statement. "Police and prosecutors are looking into what exactly happened. What's known now is that there was shooting at people sitting in a tram in Utrecht, that there were wounded, and possibly deaths."
The incident comes just three days after 50 Muslim worshippers were gunned down as they worshipped at mosques in Christchurch, by a far-right extremist.