User blog comment:Legionwrex/How did Saren reach the Citadel?/@comment-2208763-20130507064008

Considering the relay was created by the Protheans and I doubt they had vehicles to go through or there would have been evidence around the relay of the vehicles. The mass relay may have been built specifically for organic life to be transported to the Citadel. It is a plot point that has never been explained and truth be told I never thought about it until now.