US Olympic swimmers Gunnar Bentz and Jack Conger walk to the airport police station office at Rio's international airport in this still frame taken from video dated August 17, 2016, in Rio De Janeiro, Brazil. [Photo/Agencies]

RIO DE JANEIRO - Brazilian police stopped two US Olympic swimmers from boarding a flight home on Wednesday to question them about how they and two team mates were robbed at gunpoint in Rio de Janeiro at the weekend, after a judge raised doubts over their accounts.

There was confusion over the whereabouts of a third swimmer, James Feigen. One police source said he too had been stopped at the airport, but another police source later said officers had gone there to pick up Feigen, who had a reservation on the same flight, but did not find him.

Federal police also want to question US gold medallist swimmer Ryan Lochte, one of swimming's most decorated Olympians, but he had already flown home to the United States on Monday, a police spokesman said.

The four swimmers have said they were robbed by gunmen while returning to the Athletes' Village in a taxi in the early hours of Sunday after a party - an incident that stoked fears for the safety of athletes and visitors at South America's first Games.

The Rio Olympics have been dogged by a series of security scares, including the robbery of two visiting government ministers, a mugger being shot dead outside the opening ceremony and stray bullets being fired into the equestrian centre.

On Wednesday, The Guardian newspaper reported that a British team member had been held up at gunpoint while enjoying a night out in Rio in the early hours of Tuesday. The person was not seriously hurt, the newspaper added.

However, in the case of the four swimmers, police sources have said investigators have not found any evidence so far to back up their accounts.

Television images showed US Olympic swimmers Gunnar Bentz and Jack Conger walking into a police office at the airport to give testimony. A spokesman for the US Olympic Committee (USOC) confirmed the two men were taken from their flight.

"We can confirm that Jack Conger and Gunnar Bentz were removed from their flight to the United States by Brazilian authorities," said Patrick Sandusky, USOC spokesperson.

"We are gathering further information."

Earlier on Wednesday, a judge had ordered police to seize the passports of Lochte and Feigen to prevent them leaving the country, apparently unaware that Lochte had already left.

None of the swimmers face charges, the source said.

Police have taken the passports of Bentz and Conger pending their testimony, O Globo newspaper said on its web site.

People magazine reported that Lochte was spotted at an airport in North Carolina on Wednesday with his girlfriend, Kayla Rae Reid, a Playboy model. Reuters could not verify this.

Lochte's attorney did not return calls for comment.

Judicial sources said the judge would consider whether to ask Lochte to return to Brazil to give testimony or allow him to do so in the United States. He had already given testimony to Brazilian police before leaving the country.