Disgraced R&B singer R. Kelly was once worth hundreds of millions of dollars but is now serving what amounts to a life sentence in federal prison.

After decades of sex abuse allegations and an acquittal on child pornography charges, a documentary series titled “Surviving R. Kelly” finally gave his accusers a voice and helped bring down the singer. Within six months of its airing, Kelly was facing federal prosecution in New York.

He was convicted not only of sex trafficking but also of racketeering — charges that specify a person’s “enterprise” was used to carry out criminal conduct.

Sean “Diddy” Combs now faces a similar federal investigation, though the accusations against him are significantly different and it remains unclear whether they will result in criminal charges.

Authorities have said little about the probe. But law enforcement sources have confirmed to The Times that Combs is under investigation for sex-trafficking tied at least in part to civil lawsuits filed by several women who have accused him of misconduct.

Combs has denied any wrongdoing, and his attorneys have slammed the investigation as unwarranted.

After federal agents searched the artist’s homes in Florida and Los Angeles several weeks ago, his attorney decried a “premature rush to judgment of Mr. Combs” and said the investigation “is nothing more than a witch hunt based on meritless accusations made in civil lawsuits.”

Still, previous high-profile sex-trafficking cases could offer a window into how the feds typically build a case and can provide clues into what officials would need to bring charges.

“The playbook for these types of cases is R. Kelly, Jeffrey Epstein, Larry Ray and NXIVM’s founder Keith Raniere,” said Elizabeth Geddes, who delivered a six-hour closing argument in Kelly’s conviction.

In November, Combs’ former girlfriend Casandra Ventura, the singer known as Cassie, accused him in a lawsuit of rape. Within a day, he settled.

Since then, three other women have sued Combs, accusing him of rape, sex trafficking, assault and other abuses. One of the allegations involved a minor. A male producer also has sued him over unwanted sexual contact.

Geddes, who is not involved in the Combs case, said she believes Ventura might have been the trigger for the federal investigation.

She said the docuseries about Kelly spurred the Eastern District of New York to act — and that type of high-level investigation often requires an outside catalyst. In Kelly’s case, he had been acquitted in 2008 and as a result, many of his accusers lost confidence in law enforcement. But the documentary re-engaged authorities.

“Nothing puts pressure on law enforcement like a front-page story on the major newspaper in the city,” Geddes said.

Combs’ investigation, led by Homeland Security, is several months old, according to sources, and many connected to the case — including accusers and alleged witnesses — have already been interviewed.

Geddes said Homeland Security Investigations also worked the Kelly case, and its agents tend to have years of experience working with sex-trafficking victims.

She said sex trafficking requires either “force, fraud or coercion to cause a person to engage in a commercial sex act” or the trafficking of minors under 18.

“There is no statute of limitations,” Geddes said, and the key law enacted in the 2000s applies to acts from 2001 forward.

Geddes said that in addition to the sex charges against Kelly, she and her colleagues secured a racketeering indictment against the singer. The charge has famously been applied to mob bosses like John Gotti and James “Whitey” Bulger.

In racketeering cases, Geddes said, the “enterprise” carries out illegal conduct and prosecutors seek to show a broader pattern of conduct that stretches over years and involves many participants. A racketeering case also allows multiple victims’ narratives in one trial.

Racketeering became a federal crime in 1970 under the Racketeer Influenced and Corrupt Organizations Act, or RICO.

Over the years, its usage has expanded. It often is used against gangs, ranging from the Mexican Mafia to South L.A.’s Crips. Racketeering cases also have been brought against rappers associated with street gangs, including Young Thug, Kay Flock, Casanova, and Fetty Wap.

Federal prosecutors have succeeded in racketeering convictions not only against Kelly, but also against other sex traffickers, including NXIVM founder Raniere and Larry Ray, whose crimes were outlined in the docuseries “Stolen Youth: Inside the Cult at Sarah Lawrence.”

But it is unclear what evidence the feds have against Combs and whether there is enough to bring charges.

Few details are available, other than sources saying investigators left his two homes with electronics, data devices and other records.

Legal experts have told The Times that evidence in sex-trafficking cases must be extensive as such charges can be hard to prove.

“Sex trafficking for adults usually involves some sort of coercion or other restraints,” L.A. defense attorney Dmitry Gorin said. Prosecutors would need to show you “encouraged somebody to engage in sexual activity for money or some other inducement.”

Aaron Dyer, one of Combs’ lawyers, stressed in a statement released after the raids that “there has been no finding of criminal or civil liability with any of these allegations.”

The mother of Combs’ son Justin Dior Combs also slammed the investigation and the raids.

“The overzealous and overtly militarized force used against my sons Justin and Christian is deplorable,” designer Misa Hylton said after releasing video showing federal agents dressed in military gear pointing a gun at Combs’ sons. “If these were the sons of a non-Black celebrity, they would not have been handled with the same aggression. The attempt to humiliate and terrorize these innocent young Black men is despicable!”

Federal sex-trafficking and sexual assault laws also allow prosecutors to present evidence that shows a modus operandi.

“In the R. Kelly trial, several women testified about what Kelly did to them as part of a pattern of behavior. It is very much the same thing people saw in Harvey Weinstein’s prosecution,” Geddes said.

If prosecutors do file charges against Combs, they also could allege the use of forced labor under threat, Geddes said. Ventura, Combs’ former girlfriend, alleged she was forced into sex acts with other men and suffered physical harm for complaining. If true, this could be considered forced labor, Geddes said.

Kelly was convicted of eight counts of the Mann Act, which was passed in 1910 and sought to criminalize what’s now known as human trafficking. The law initially banned transporting a woman or girl across state lines “for the purpose of prostitution or debauchery, or for any other immoral purpose.”

The Mann Act now covers transportation across state lines “with [the] intent that such individual engages in prostitution, or in any sexual activity for which any person can be charged with a criminal offense.”

In the allegations against Combs, one woman said she was brought from Detroit as a 17-year-old to Combs’ studios so he could rape her along with his cohorts, Geddes said.

Before the highly publicized searches of Combs’ properties were executed, Geddes said, prosecutors and HSI agents had to “have made some headway into the investigations.”

“What we can say at this stage is there was enough probable cause to convince a magistrate to issue a search warrant,” she said. “Before getting such a warrant, agents have typically interviewed multiple witnesses.”

Geddes said