Football star has insisted he is unconcerned by the inquiry because he is innocent.
US police are seeking a DNA sample from Cristiano Ronaldo over accusations that he raped a woman a decade ago in Las Vegas.
Ronaldo, 33, widely regarded as one of the greatest football players of all time, has maintained he is innocent of rape and said through his Las Vegas lawyer on Thursday that the sexual encounter in question was consensual.
However, Nevada police have made an official request to counterparts in Italy for a sample of the footballer's DNA.
The matter came to light in September 2018 when Kathryn Mayorga sued Ronaldo in Clark County District Court in Nevada, accusing the athlete of raping her in a Las Vegas hotel penthouse suite in 2009, then paying her $375,000 in hush money.
Mayorga's lawyers said in September she was now seeking $200,000 in damages and to void the non-disclosure agreement, which they said she was coerced into signing by "personal reputation protection specialists hired" by the football star.
Ronaldo said on Twitter after the suit was filed that his "clear conscience" would allow him "to await with tranquility the results of any and all investigations."
The Las Vegas Metropolitan Police Department said at the time an incident report was filed with police on the night of the alleged assault, and the case had since been reopened. But the police declined at the time to name Ronaldo as a subject of the investigation.
The Wall Street Journal, citing an unnamed law enforcement official with knowledge of the case, reported on Thursday that authorities in Las Vegas had sent a warrant seeking DNA from the football star to the court system in Italy, where Ronaldo plays as a forward for the professional football club Juventus. He also is captain of Portugal's national team.
According to the Journal, investigators want to see if Ronaldo's DNA matches a sample found on a dress of his accuser that belongs to someone other than her.
Responding to a Reuters query about the report, Laura Meltzer, a Las Vegas police spokeswoman, said in an email the department is "taking the same steps in this case as in any other sexual assault to facilitate the collection of DNA evidence. We can confirm that an official request has been submitted to Italian authorities."
In response to a similar query, Ronaldo's lawyer Peter Christiansen appeared to confirm the DNA request in a separate statement.
"Mr. Ronaldo has always maintained, as he does today, that what occurred in Las Vegas in 2009 was consensual in nature, so it is not surprising that DNA would be present, nor that the police would make this very standard request as part of their investigation," the attorney said.