By Cassandra Garrison and Caroline Stauffer

MIAMI/RIO DE JANEIRO (Reuters) - A U.S. Olympic swimmer has accused his team mate, Ryan Lochte, of playing a key role in an incident that has tainted South America's first Games, saying he tore a poster off a wall and argued with armed security guards at a Rio gas station.

In his first account of last week's incident, the youngest of the four swimmers involved, Gunnar Bentz, 20, said the guards confronted them after they had urinated behind some bushes and Lochte tore a metal-framed advertising poster from a wall.

It is the first time any of Lochte's companions has spoken publicly of the 32-year-old's role in the incident, and it contradicts Lochte, one of America's most decorated swimmers, who told U.S. television that he and his team mates were robbed at gunpoint.

Brazilian police have denounced Lochte's account as a fabrication designed to cover up for the group's bad behavior after a night spent partying until dawn, prompting apologies from both Lochte and the U.S. Olympic Committee this week.

Bentz said he, Lochte and two other swimmers - all gold medalists at the Rio Games - had been in a taxi when they stopped at the gas station to use the bathroom, contradicting Lochte's initial account of being pulled over by gunmen posing as police.

"There was no restroom inside, so we foolishly relieved ourselves on the backside of the building behind some bushes," Bentz said in a statement issued via the University of Georgia, where he attends college.

"I am unsure why, but while we were in that area, Ryan pulled to the ground a framed metal advertisement that was loosely anchored to the brick wall."

Armed security guards then challenged the swimmers and one of them drew a gun and demanded they sit down, but Lochte then stood up and yelled at one of the guards, Bentz said, adding that he and team mate Jack Conger had tried to restrain him.

"After Jack and I both tugged at him in an attempt to get him to sit back down, Ryan and the security guards had a heated verbal exchange, but no physical contact was made," Bentz said.

'THE UGLY AMERICAN'

The U.S. Olympic team has sought to draw a line under the scandal which has embarrassed the host city and angered ordinary Brazilians who jeered Bentz and Conger, calling them "liars" and "fakes" as they made their way out of Brazil on Thursday.

Bentz, who said he never gave a false account to police, and Conger arrived in Miami on Friday on an American Airlines flight. They sat in curtained-off seats for much of the journey.

The fourth member of the group, Jimmy Feigen, was set to leave the country on Friday night after agreeing with a judge to donate $11,000 to a sporting charity, a condition of police returning his passport.

Lochte returned to the United States on Monday, a day after the incident and before police denounced his version.

In his apology, Lochte said he should have been more careful and candid in his account but it had been traumatic to have a man point a gun at him in a foreign country and demand money.

"I want to apologize for my behavior of last weekend," Lochte said as a social media storm gathered over him at home and in Brazil. He did not say why he embellished details of the encounter, nor why the act of vandalism occurred.

"Regardless of the behavior of anyone else that night, I should have been much more responsible in how I handled myself and for that I am sorry to my team mates, my fans, my fellow competitors, my sponsors and the hosts of this great event."

U.S. tabloid press dubbed him "The Ugly American" and "The Lochte Mess Monster", turning on the once-beloved champion and saying his behavior fed the worst stereotypes of Americans abroad.

The incident followed a series of muggings and armed robberies of high-profile athletes and visitors in Rio, including two government ministers - security scares that detracted from the image Games organizers hoped to portray.

The International Olympic Committee said on Friday it had set up a disciplinary commission to investigate the incident and could sanction the swimmers if they considered their behavior violated the Olympic charter in some way.

The U.S. Olympic Committee and USA Swimming have said they will consider whether to take action against the swimmers. Lochte has said that he hopes to represent the United States at the 2020 Tokyo Olympics.

(Additional reporting by Rodrigo Viga, Paulo Prada, Caio Saad, Brad Brooks, Brad Haynes and Liana B. Baker; Writing by Daniel Flynn; Editing by Alison Williams, Mark Bendeich and Mark Lamport-Stokes)