Verified answer

Check it out:

http://en.wikipedia.org/wiki/The_Death_of_Superman

in the early 90's the comics created the Villain Doomsday who beat the JLA senseless, killing/maiming several members

Superman is called in, and battles the beast to essentially a dual killing strike (Doomsday was killed, but it is his nature that he's reborn after each death and thus forth immune to that method of death), after the real Superman returned, it was explained he was just knocked the F out, but because no one understood his biology, they thought he was dead

Doomsday killed him several years back. Don't worry, he's back.