The Public Prosecutor's Office says five Rotterdam police officers who fired shots into a crowd of partygoers last month are being treated as suspects. It was not specified what offences they are suspected of.

The officers said they felt threatened by a rioting crowd of at least 15,000 at the free party on Hook of Holland beach, and had to fire warning shots to keep attackers at bay. Many of the partygoers were recognised as hooligans from Rotterdam who turned against the police. One of them, 19-year-old Robby van der Leeden, was killed by a police bullet. Scores of people were injured in the throng.

Six party visitors are under suspicion of inciting the riots, two of them are still in detention. An independent investigation into the incident has been ordered by local authorities.