Bridge across the Rio Grande Puzzle

There's a bridge across the Rio Grande connecting Mexico and the US.
The Americans have a watchtower on the Mexican side. There is a guard who has strict orders to shoot anyone leaving Mexico and turn back anyone trying to enter Mexico without a pass.
The sentry comes out every 3 minutes with a rifle and a megaphone to check the bridge and carry out his orders.
It takes exactly 6 minutes to cross the bridge.
There is no check on the US side.
How does cunning Mrs Gonzales manage to go across to the US without getting shot ?

Click here for Answer

She will walk on the bridge towards US for 3 minutes and just as the guard was about to come out, she will turn around walking back to Mexico. The guard will see her and demand for a pass. Since she didn't have it will be sent back to US and she will go to US without getting shot.