In my opinion, the correct choice in this particular case is past simple.
The reason is that the stabbing is described as a separate event that took place on the night of October 1. It is not a simple background of the arrest.
Another way to look at it is that past perfect describes a state of completeness in the past, rather than an event in the past. If you say "he had been stabbed six hours ago", it means that six hours ago he was already wounded; the stabbing supposedly occurred at some unspecified time before that.
Except of conditional statements (which sometimes use past perfect for another reason), past perfect almost always carries a sense of "already". In the text you quoted, I think this "already" would not make too much sense, so past perfect should not be used.
Looking at the comments, I realize that some speakers would use the perfect in this case, and it would probably be understood just the same. However, consider good rule-of-thumb defined in the Canonical Post about the Perfect is "Don’t use the perfect unless you need it." There is no hard-and-fast rule, but look at that post to see how sentences can sometimes be formed with past simple and still convey the order of events.
In my opinion, the given sentence is clearly understood using past simple, and the past perfect is not needed.
As a counter-example, I think it would be adequate to use the perfect to describe the actions/events of the suspects that directly led to the arrest, such as
The four persons who were arrested by the police on Tuesday had been under surveillance by the police for several weeks.