Can You Solve The Bridge Crossing Riddle?

Riddle: In the early 1900's in England, there was two towns: town A and town B, and there was a huge river in between them. The only way across to each town was by a long bridge with a guard post in the middle. It took 20 minutes to cross the bridge from one side to the other and 10 minutes each way from the guards tower. There were no hiding spots on the bridge so people couldn't sneak past, and the guard comes out every 5 minutes or so to check to see if anyone is trying to cross. The guards tower was placed because of a law that stated that no one was aloud to leave their own town into the other because of political reasons and anyone who was caught by the guard would be fined and told to turn back. Many people have tried to cross but have always been caught, even very fast runners have only able to make it past the guards tower before being caught. But one night someone was able to make it across....How did they do it?