RMS Lusitania was torpedoed by the German submarine U-20, there has never been much debate about that since there were witnesses on both sides.
What was debated, was the source of a second explosion- whether it was ammunition, coal dust, or a second torpedo.
With the state of technology at the time, if I was commanding a U-boat, and presented with a target like Lusitania, steaming straight in my direction, not zig-zagging, there is no way I would have only fired one torpedo.
The target was just too valuable to take a chance at firing just one weapon, and even if hit, the damage could not be depended on to be fatal.
I would also not want to risk a second shot so close to an enemy harbor where naval forces would likely be based.
There still may have been ammunition aboard, but it is unlikely the U-boat only fired one weapon.