12/11/2012 09:39 EST | Updated 02/10/2013 05:12 EST

Dutch police arrest 4 more, including first adult, in deadly assault on football linesman

THE HAGUE, Netherlands - Dutch police have arrested four more suspects in their investigation into the attack on a volunteer linesman who died a day after players in a youth team assaulted him after a match.

The arrests on Tuesday bring to eight the number of suspects in custody in the case — seven of them teenagers.

Police say in a statement they have arrested three teenagers and a 50-year-old man, the first adult to be detained in the case, all from Amsterdam.

Linesman Richard Nieuwenhuizen's death just over a week ago sparked national soul-searching about the rising tide of violence on and around Dutch sports fields and how to rein it in.

Some 12,000 people attended a silent march on Sunday to commemorate Nieuwenhuizen.