It was supposed to be a celebration to mark the opening in Berlin of Europe’s biggest train station. It turned into scenes of chaos, however, when a teenager ran amok with a knife, stabbing 28 people, leaving six in a serious condition.Police say one of the first victims was HIV positive, raising fears the others may have been infected. The German Chancellor Angela Merkel attempted to reassure the public about security, just two weeks before the country hosts the football World Cup.She says never before have security policies between the federal and local governments been so coordinated. The alleged attacker is a 16-year-old who has a police record for assault. Police are still trying to work out the motive but have ruled out a far-right link.