What did the Germans sink in 1915 that killed 128 Americans?

The Germans sank a British passenger ship, the RMS Lusitania, with a total death toll of over 1500. A German submarine, the U-20 did this, firing a torpedo, without warning, while submerged and unseen, which seemed extra sinister. It was more than fifty years before the British government revealed that the Lusitania had in fact illegally been carrying contraband munitions of war, millions of rounds of rifle ammunition and tons of explosives tot he British military, which would have justified the Germans in sinking her.