Tony Stark's Accident

Some sources say that the shrapnel in his heart were caused by falling in a trap (http://marvel.wikia.com/Iron_Man_(Anthony_%22Tony%22_Stark) ) and some say that it was from the general explosion (http://marvel.com/universe/Iron_Man_(Anthony_Stark) ). Does anyone know which was the actual cause? Might he have fallen into the trap because of the explosion? I'm confused :/

It has changed over the years. Now, I believe it was that he was helping transport some mines and one of them blew up during a raid.

Yeah, or that he was attacked.