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Conor McGregor, the Ultimate Fighting Championship’s biggest star and one of the world’s highest-paid athletes, is under investigation in Ireland after a woman accused him of sexual assault in December, according to four people familiar with the investigation.

McGregor has not been charged with a crime. Following the usual protocol in criminal investigations in Ireland and much of Europe, where a formal charge does not necessarily follow an arrest, McGregor was arrested in January, questioned by law enforcement authorities and released pending further investigation, according to the people.

The allegations have not been proved, and the fact that an investigation is continuing does not imply that McGregor is guilty of a crime. A lawyer for McGregor in Dublin did not respond to messages seeking comment.

Today, McGregor announced his retirement from UFC, though a spokeswoman said it was unrelated to the investigation.

“I wish all my old colleagues well going forward in competition,” he wrote on Twitter. “I now join my former partners on this venture, already in retirement. Proper Pina Coladas on me fellas!” (Proper No. Twelve is a whiskey brand he founded.)

McGregor has fought only once since 2016, a bout in October that he lost to Khabib Nurmagomedov, and he has previously announced a retirement, only to come back.

The woman making the allegation said it had occurred at the Beacon Hotel, an establishment attached to a business park on the edge of Dublin. There is little to suggest it would be a haunt for one of Ireland’s best-known sports figures, a multimillionaire with a loyal, global fan base. McGregor is an occasional guest there, usually booking its sole penthouse room, according to a person with knowledge of the situation. The last and most recent visit occurred in December.

Irish media have reported on the case since news of the assault broke late last year, but without naming McGregor. Laws in Ireland restrict the media from identifying individuals charged with rapeunless they are convicted, which has not happened in this case. News outlets reporting the identity of a suspect before a charge is brought often face costly libel and breach of privacy lawsuits in Ireland. Publication after a charge is filed could lead to a more serious contempt of court indictment.

A spokesman for Ireland’s police service, known as the Gardai, would not confirm if McGregor is the suspect. In response to a request for comment related to an “unnamed sportsman,” it said a man was arrested Jan. 17 and released without charge while investigations continued.

“Investigations are ongoing in this case and at this time a file continues to be prepared for the Director of Public Prosecutions,” a police statement said.

McGregor and the UFC have not commented on the allegations. Karen J. Kessler, a publicist for McGregor based in New Jersey, issued a statement that did not address the validity of the accusation, but asserted that his retirement had nothing to do with the investigation.

“This story has been circulating for some time, and it is unclear why it is being reported now,” the statement said in part. “The assumption that the Conor retirement announcement today is related to this rumor is absolutely false.”

The Beacon is about a 25-minute drive from Crumlin, the working-class South Dublin neighborhood where McGregor grew up. The hotel’s typical guests are white-collar workers tied to the financial services or technology companies that have recently relocated to the area. Still, people familiar with the hotel operations, speaking on condition of anonymity, said McGregor had visited before the night police say the incident took place. He booked the hotel’s penthouse, the only two-room suite in the facility.

Police investigating the attack retrieved evidence from the room McGregor stayed in and also secured closed-circuit camera footage, according to a person with knowledge of the investigation.

The limitations on naming suspects and McGregor’s wealth — he made an estimated $99 million last year, according to Forbes — have created an unusual dynamic in an era when celebrity scandals are usually the subject of fervent media scrutiny. An investigation into the possible involvement of soccer star Cristiano Ronaldo in a sexual assault, reported to police in Las Vegas in 2009, has become an international spectacle.

In the McGregor case, some newsrooms in Ireland have barred employees at meetings from even mentioning his name in connection with the case. Managers have asked staff members to refer to him as the “famous sportsman,” according to a journalist at one of Ireland’s national newspapers.

An internal memo from broadcaster RTE leaked into the public domain after the arrest.

The document, which appears to be the schedule for a morning radio show, was stamped “not for publication/broadcast” in capital letters and was printed Jan. 18. It explained that McGregor had presented himself at a police station at 5 p.m. a day earlier. On the morning the memo was printed, RTE news bulletins included the story of the man being questioned but did not name him. RTE declined to comment.

The document is one of many messages linked to the case that have been spread widely on social media, via messaging applications as well as internet forums.

McGregor’s rapid rise from a destitute mixed martial arts fighter living with his mother to the UFC’s most-prized asset has become one of the biggest stories in Ireland in the past decade. McGregor, who goes by the nickname Notorious when fighting, has largely enjoyed the attention, regularly taking to social media to brag and show off the trappings of his newfound wealth.

McGregor shot to wider fame in 2017 when he participated in one of the most lucrative boxing matches ever, losing to the undefeated champion, Floyd Mayweather Jr.

He has also been in the headlines for the wrong reasons, courting controversy since he rose to prominence four years ago. In July, he pleaded guilty to a charge of disorderly conduct, after he threw a dolly at a bus during a promotional appearance at Barclays Center in Brooklyn, New York. Two fighters were injured by shattered glass.

In January, the Nevada Athletic Commission suspended McGregor from fighting for six months and fined him $50,000 for his role in a brawl that took place after his loss to Nurmagomedov in October. Nurmagomedov was barred for nine months and fined $500,000 for his role in the melee.

On March 11, McGregor was arrested in Miami Beach and charged with robbery and criminal mischief after he was accused of stealing a cellphone from someone trying to take his picture. His lawyer described the altercation as minor and said McGregor, who was released after posting bail, would cooperate with police.

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