Rio de Janeiro: A US 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 US 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 behaviour after a night spent partying until dawn, prompting apologies from both Lochte and the US. Olympic Committee this week.

Bentz said he, Lochte and two other swimmers — all gold medallists 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.”

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 apologise for my behaviour 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.