Firn,

On Twitter there were several references to this man being involved a random act of violence, possibly with a knife and he was released two weeks ago after serving an incredibly short jail sentence.

On my second reading The Guardian does refer to this:
where he was serving a two-year sentence for stabbing a 19-year-old man on Copenhagen’s inner-city train system. He had been released from prison only two weeks before the attacks in Copenhagen this weekend.
Wiki has this:
he only served one year out of his two year prison sentence