Complex sentence: The gunmen shouted, "Is this" Charlie Hebdo "?!", and after realizing their mistake left for the magazine's headquarters at number 10 Rue Nicolas-Appert.
Split into simpler sentences: The gunmen shouted, "Is this" Charlie Hebdo "?!", before realising they had the wrong address and leaving. They then went to the magazine's headquarters at number 10 Rue Nicolas-Appert.