Four US Olympic swimmer fabricated a story about being robbed at gunpoint in Rio de Janeiro with teammates after an altercation at a petrol station, Brazilian police have said.

Authorities began investigating the incident after the four swimmers, including gold medallists Ryan Lochte and Jimmy Feigen, said they had been robbed by gunmen impersonating police officers in the early hours of Sunday.

"There was no robbery," Fernando Veloso, the head of Rio's civil police, told reporters on Thursday. "There was no robbery of the kind reported by the athletes."

Earlier on Thursday, a Brazilian news channel aired a CCTV video from the petrol station showing the swimmers involved in a dispute with employees, over what security sources said was damage to the premises.

The images broadcast on Globo TV appeared to show the athletes being prevented from leaving the station by security.

The video does not show them causing any damage, but only being hussled out of the bathroom by uniformed employees of the Shell station.

Brazil's G1 site quoted Rio police saying that the Olympians, who had been out partying, stopped at the petrol station in a taxi to use the bathroom.

They broke the bathroom door and tried to leave without paying for the damage, but a security guard pulled out his weapon and made them stay until police arrived, prompting a confrontation, G1 reported.

Speaking to reporters, police chief Veloso accused the swimmers of vandalism, without giving any further details.

He said the swimmers had offered to pay some money for damage to the gas station, adding that police were considering whether to recommend charges against any of them.

'Get down'

Lochte originally said that he and his teammates - Feigen, Jack Conger and Gunnar Bentz - were robbed by someone claiming to be a policeman early on Sunday.

One of the best-known faces of the US Olympic squad, Lochte said the four were held up by muggers posing as police as they left a late-night party in central Rio.

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He said he had a gun put to his forehead after the swimmers' taxi was pulled over by criminals who forced them to lie on the ground and stole money and other items.

"The guy pulled out his gun, he cocked it, put it to my forehead and he said, 'Get down'," Lochte said afterwards. "He took our money, he took my wallet - he left my cell phone, he left my credentials."

The report caused the Olympic authorities huge embarrassment, highlighting security worries at an Olympics where Brazil has deployed 85,000 police and soldiers - double the number used at London 2012.

Detained at airport

However, police soon raised doubts over the veracity of the allegation and the swimmers could face charges of filing a false claim.

Lochte is in the US, while Conger and Bentz were detained by Brazilian police late on Wednesday just as they were about to take off from the international airport for home.

Al Jazeera's Gabriel Elizondo, reporting from Rio de Janeiro, said it was probably "going to be up to a judge" to decide if any charges will be pressed against the US athletes.

"So far, three of the four Olympic swimmers remain here in Rio de Janeiro; their passports have been confiscated by a judge's order until a final ruling is decided."

Rio Games organisers on Thursday defended the four swimmers, saying they were just "kids" who were having fun and 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 said.

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