Three people were killed following a shooting on a tram in the central Dutch city of Utrecht, the city's mayor says.
Nine others were injured in the incident, which police said appears to be a terrorist attack.
Police are looking for a 37-year-old Turkish man named as Gokmen Tanis and have warned people not to approach him.
Schools have closed and security has been increased while counter-terrorism police work to find the gunman.
"Our country today has been jolted by an attack," Prime Minister Mark Rutte said on Monday. "Police and prosecutors are looking into what exactly happened."
"What's known now is that there was shooting at people sitting in a tram," he added. He described the attack as "deeply disturbing".
"We cannot exclude a terrorist motive," Dutch anti-terrorism co-ordinator Pieter-Jaap Aalbersberg told a news conference on Monday.
He added that there had been shootings at "several locations", but did not elaborate on where these were.
Counter-terrorism police earlier surrounded a house near the 24 Oktoberplein junction, where the tram attack took place, but no arrests appear to have been made.
A local businessman told BBC Turkish that the suspect had previously fought in Russia's republic of Chechnya.
Jihadist groups, including those aligned with the Islamic State (IS) group, have long operated in the region.
"He was arrested because of his connections with [IS] but released later," the businessman told the BBC.
Meanwhile, the threat level has been temporarily raised to its highest point in the province of Utrecht.
Utrecht University has closed all of its buildings and trains are not allowed to run into the city's central station.
People have been told to stay indoors and paramilitary police have been sent to airports and mosques amid increased security concerns.
Utrecht is the Netherlands' fourth largest city and has a population of around 340,000.
Crime levels are low and gun killings are rare, which is the case for much of the country, reports BBC.