Generally if you are suspected of comitting a sufficiently serious crime -- where the prison term would be 18 months and above -- you can be remanded if there's a suspicion you are going to interfere with the investigation or flee.

The prosecutor requests the remandment, and a judge has to approve within a short period after the arrest (or release the suspect).

Isolation here can be requested if there's a suspicion the alleged criminal can interfere with the investigation, warn others, destroy evidence etc. by communicating.

You can try Google translate on http://www.themis.dk/searchinclude/Lovsamling/Retsplejeloven... if you want to know more.

This can in theory go on for 6 months, if the alleged crime would be penalized with less than 6 years in prison, or a 1 year if 6 or more, if the court approves the increase every 4 weeks.

As a Danish person I was also confused about that when I first read it. I think it's a misuse of the word "warrant", and more has to do with whether or not they've charged him (I think).