I was told that a knot complement in $S^3$ is an EilenbergMc Lane space. And that it is quite easy to see this. However I am not able to find out why. could you help? Thanks
Papakyriakopoulos, C. D. On Dehn's lemma and the asphericity of knots. Ann. of Math. (2) 66 (1957), 1–26. 

