He said top Dutch officials were meeting to try to determine the motivation behind the incident. The alert will be in place until at least 18:00 local time.
Turkish intelligence services are investigating the deadly shooting attack in the Netherlands to determine whether it was a terror attack or a possible family dispute, President Recep Tayyip Erdogan said Monday.
Police, including heavily armed officers, flooded the area after the shooting that happened Monday morning on a tram at a busy traffic intersection.
HLN reports that security has also been tightened at train stations and mosques in Rotterdam, but it is now unclear why mosques have been specifically mentioned.
Dutch Prime Minister Mark Rutte convened crisis talks immediately after the incident, which came three days after a lone gunman killed 50 people in mass shootings at two mosques in the city of Christchurch, New Zealand.
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Update 2:10 p.m. Eastern: Police confirm that the main suspect - whose photo they earlier released - has been taken into custody.
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Prestigiacomo directed attendees to a voter registration table and encouraged those who could vote to make their voices heard. We must teach students more than just how to pass tests ... "We shouldn't really have to be doing this".
Utrecht's transport authority said tram services in the city have all been cancelled.
Pieter-Jaap Aalbersberg said in a statement that the "threat level has gone to five, exclusively for the Utrecht province", referring to the highest level.
City authorities told residents to stay inside while the shooter or shooters were on the loose.
Police quickly moved in to cordon off the area even as a government spokesman said one person was killed in the attack. "Several trauma helicopters have been deployed to provide help", the police added.
Utrecht, the Netherlands' fourth largest city, has a large student population and is known for its picturesque canals.
According to Dutch broadcaster RTV Utrecht, one eyewitness said he heard several shots fired and saw one woman lying on the ground.