What country sank the lusitania in 1915 and why did this upset the united states?

Germany sank the ship Lusitania in 1915 by a torpedo from a U boat of Germany as they thought it is carrying war supplies from America to England. But It actually had 1900 passengers in it out of which 1158 were killed in the attack. 148 American were killed as the ship sank. Upset with the deaths the opinion of American public changed on Germany and its allies for worse. Yet as USA was very reluctant to join the world war 1, Then United States president sent diplomatic notes to Germany stating that United states citizens have the right to travel safely in Sea waters and warned hence forth sinking of ships will be considered "deliberately unfriendly."