U.S. Swimmers Damaged Gas Station, Guard Forced Them to Pay Up

9:08 AM PT -- Security camera footage from the gas station shows the swimmers walking through a hallway back toward the bathroom, and then leave. You can't see what happens by the bathroom door.

Once they get in their taxi though, gas station employees approach and after some discussion they all get out of the cab. At the very end of the footage ... the swimmers have their hands up and it appears they're being forced to sit on the ground.

There's no gun visible in the video.

Ryan Lochte and his teammates left out a huge part of their "robbery" story -- which is that they busted up a gas station bathroom, and THEN a security guard pulled a gun on them ... TMZ Sports has learned.

Sources connected to James Feigen tell us the real deal is ... the 4 swimmers -- including Jack Conger and Gunnar Bentz -- were on their way home from a party early Sunday morning when they stopped to take a leak at a gas station. We're told one or more of the guys was peeing outside the bathroom, and also kicked in the bathroom door.

Our sources say the swimmers were all wasted.

That's when the gas station's armed security guard rolled up, pulled his gun and demanded they pay for the damage. We're told he did take money from the swimmers, but only to cover the expenses, and then let them go.

Rio police say the gas station manager tells a slightly different story -- claiming the guard never pulled the gun and the manager merely asked them to pay, which they did.

Bottom line ... it's possible Lochte and co. didn't lie about a gun or losing money. However, they DID create a whole bogus scenario about their taxi getting pulled over and fake badges -- and totally omitted the part about them starting the trouble in the first place. On Wednesday night, Lochte repeated most of those lies to Matt Lauer.

They could face prosecution for lying to Rio cops. They have a news conference scheduled for later Thursday.