Brazilian police are considering charging a group of US Olympic swimmers with vandalism and giving false testimony, Rio's chief police has said.

He accused the swimmers of lying about an incident that has marred South America's first Olympic Games.

Three of the four swimmers involved in the incident at a Rio gas station are being prevented from leaving the country pending the outcome of the police investigation.

The fourth, gold medalist Ryan Lochte, returned to the United States on Monday.

"In theory, they could be held responsible - by they, I mean one or two or all four of them - with falsely reporting a crime and vandalism," civil police chief Fernando Veloso told a news conference.

He said neither offence was punishable by prison.

"There was no robbery as the swimmers described it," he said.

Police began investigating the incident after Lochte told US television they had been robbed by gunmen impersonating police officers who pulled over their taxi in the early hours of Sunday, as they returned to the Athletes' Village from a party.

Their swimming competition had finished on Saturday evening.

Veloso said investigations had revealed that the swimmers' taxi had pulled into the station where they behaved in a hot-tempered way and damaged the station's bathroom.

They broke a mirror and a soap-holder, he said, adding that they then handed over a total of $20 in US currency as compensation.

According to Lochte's account, $400 was stolen from them.

At one point, a security guard pulled a firearm after one swimmer behaved erratically, Veloso said, adding that the guard had not over-reacted: "From the moment the gun was pulled out, they calmed down. Once they were calm, the gun was lowered."

Earlier, Brazilian TV aired a video that showed the swimmers did not tell the whole truth in their original accounts.

The security-camera images broadcast on Globo TV appeared to show the swimmers - who also included another gold medalist, Jimmy Feigen, as well as Gunnar Bentz and Jack Conger - in a dispute with staff at the gas station, a fact police say they did not mention in their accounts.

The video does not show them causing any breakage, but only being hustled out of the bathroom by uniformed employees.

Security guards then prevented the swimmers from leaving in a taxi and the Americans appear to offer them money from their wallets.

Three of the swimmers are made to sit on the ground with their hands in the air.

At one point, Lochte stands and appears to argue with the guards but is made to sit down again.

On Sunday, Lochte had told NBC that the taxi he was travelling in with his three team mates was flagged down by robbers posing as police and they held a gun to his head during a robbery.

He made no mention of stopping at a gas station.

NBC host Matt Lauer said yesterday that Lochte repeated a slightly modified version to NBC in an interview not yet aired, saying the swimmers had stopped at a gas station and that a gun was pointed in his direction during the robbery.

A spokesperson for the US Olympic Committee (USOC) declined to comment on the video footage.

The USOC has said that the three swimmers who remain in Brazil would be helping police with their investigation, after authorities had stopped Bentz and Conger from leaving the country the previous day and seized their passports.

Bentz and Conger arrived at a police station in downtown Rio earlier today to speak with investigators.

Rio Games organisers have defended the four swimmers, saying they were just kids who made a mistake.

"These kids tried to have fun, they tried to represent their country to the best of their abilities," Rio 2016 spokesman Mario Andrada told reporters, without elaborating.

"They competed under gigantic pressure. Let's give these kids a break. Sometime you take actions that you later regret. They had fun, they made a mistake, life goes on."