Nine people were injured when the roof of a party boat collapsed in Berlin’s central Mitte district, the fire brigade reported on Tuesday evening. Four of them were seriously injured, a fire brigade spokesman told dpa, while five other guests sustained minor injuries. A rescue helicopter landed at the site. About 100 people were celebrating on board the boat. The cause of the accident is still unclear, according to the fire brigade spokesman. The accident occurred on an event boat on the River Spree, which flows through the the German capital. The fire brigade said that a large number of its p…