U.S. Swimmer: Video Reportedly Shows Brawl with Security At Gas Station

An American swimmer was allegedly caught on video breaking down a bathroom door at a gas station and fighting with a security guard in Brazil ... a police source tells ABC News.

The video is part of the investigation into Ryan Lochte and 3 other U.S. swimmers who claim they were robbed at gunpoint early Sunday morning.

The police source claims the gas station fight was captured on surveillance video -- though it's unclear which swimmer was fighting.

Local reports in Brazil say the swimmer was going bersek in the bathroom -- and the security guard, who worked for the gas station, confronted him ... and the two began fighting.

A source connected with one of the swimmers tells TMZ ... the scenario described in local Brazilian reports is "absolutely accurate."