Police are continuing to investigate the impact of a data leak last week in which hackers obtained the “work-related contact data of all police officers”.

The Dutch national police force employs some 62,000 officers. According to an email sent to staff at the weekend, police chief Janny Knol an “office account” was hacked revealing names, email addresses and phone numbers.

How the hackers gained access is still unclear. It is possible that one of the officers fell for a phishing scam, director of security firm ESET Dave Maasland told the NRC. But it could be a “clever hacker who may or may not be in the Netherlands” or even “a foreign government”, he told the paper.

Maasland said police cyber security is “complex”. “They have to defend themselves against bored teenagers, organised crime and sophisticated hacks from abroad. But the conclusion has to be that they have to improve their security.”

The stolen information has not turned up anywhere yet, but the incident has caused much unrest among officers, particularly among those involved in covert operations, although most of their actions have been anonymised.

“Officers on the beat are most worried,” chair of police union NPB Nine Kooiman said. “They are afraid their data will be shared online and that they will become the victims of doxing,” she said. So-called Romeos (non-uniformed police officers who mingle with violent demonstrators) are particularly worried about harassment.

Police are not releasing much information about any progress they have made in the case for fear of telling criminals more than they already know.

“We need to take this very seriously. This is not a prank or a hobby and neither were the police negligent,” a police source told the paper.