By Daniel Flynn

RIO DE JANEIRO (Reuters) - Brazilian police searched on Monday for the driver of a taxi in which U.S. gold medalist swimmer Ryan Lochte and three team mates were robbed at gunpoint, as authorities stepped up security in Rio de Janeiro after a spate of incidents.

The swimmers were returning to the Athletes' Village in the early hours of Sunday after a party hosted by the French Games delegation when armed men carrying police badges pulled them over. The gunmen ordered them to drop to the ground and demanded their wallets and belongings, Lochte said.

When he refused, one of the men cocked a pistol and placed it against the swimmer's head. He then handed over his wallet.

"We apologize to those involved and once again we regret the fact that violence is an issue in these Games," said Mario Andrada, a spokesman for the Rio 2016 organizing committee.

"We have requested that authorities make sure that people are safe everywhere in the city."

There was initially confusion over the incident.

International Olympic Committee spokesman Mark Adams told a news conference that the U.S. Olympic Committee (USOC) had denied the robbery took place. However, the USOC swiftly did an about-turn and confirmed it took place.

Organizers suggested it had been difficult to confirm the information early on Sunday because the swimmers were still asleep after the ordeal.

Police said on Sunday they were investigating the incident, which has raised concern over the safety of competitors and fans in Rio. The city of more than six million people has a long history of violent crime linked to the impoverished slums that cling to the hills overlooking its golden beaches.

"The police spoke to some of the athletes yesterday and the police are looking for the cab driver who drove them back to get more information," Andrada said.

Thirty-two-year-old Lochte, among the most successful male swimmers in Olympic history, said he had initially refused to get to the ground when ordered by the men.

"I was like, we didn't do anything wrong, so I'm not getting down on the ground," Lochte said in the interview broadcast on Monday on the "Today" show. "And then the guy pulled out his gun, he cocked it, put it to my forehead."

At that point, Lochte said he complied.

"I put my hands up, and was like, 'Whatever.'"

The gunmen stole their cash and wallets, he said. They let the swimmers keep their cell phones and credentials.

Lochte has won a career 12 Olympic medals, six of them gold, behind team mate Michael Phelps's 28 medals, including 23 golds.

The robbery of one of the Games' most celebrated athletes - together with team mates Gunnar Bentz, Jack Conger and Jimmy Feigen - has fed concern over safety at the first Olympic Games in South America.

In addition to the swimmers, Swedish tourists were briefly abducted when they visited a slum, Portugal's visiting education minister was robbed at knife point, bullets flew into the equestrian center and a Games bus was attacked with stones.

Police shot dead a man outside the Games opening ceremony in the Maracana stadium when he attempted to rob a bystander.

With 85,000 police and soldiers deployed to provide security, roughly twice the number for London 2012, authorities have said they can ensure the safety of everyone at the Games.

After months of concerns in the run-up to the Games over late delivery of infrastructure, the Games themselves have been plagued by logistical problems, including poor signage, delays to transport links and the discoloration of the diving pool.

On Monday, an overhead TV camera fell to the ground in the Olympic Park, slightly injuring two people.

(Additional reporting by Laila Kearney; Editing by Bernadette Baum and Mark Bendeich)