Two men have been charged with providing fentanyl-laced heroin that killed Cecilia Gentili, a prominent transgender activist and actress who was found dead in her Brooklyn home in February.

An indictment unsealed in Brooklyn federal court on Monday accused Michael Kuilan, 44, and Antonio Venti, 52, of supplying Ms. Gentili with the drugs. It was the first time that officials have disclosed Ms. Gentili’s cause of death.

Both men pleaded not guilty before Magistrate Judge Lois Bloom in Brooklyn federal court on Monday afternoon.

Ms. Gentili was a well-known community leader, activist and actress on the critically acclaimed television show “Pose.” Her death, at age 52, was met with an outpouring of grief from the L.G.B.T.Q. community, and she was mourned by New York elected officials, including Gov. Kathy Hochul, Mayor Eric Adams and Representative Alexandria Ocasio-Cortez.

Her life was celebrated with an energetic and politically charged funeral at St. Patrick’s Cathedral, the seat of the Roman Catholic Archdiocese of New York, which subsequently denounced the event after fiery backlash from conservative news media over the comportment of some of the mourners.

Breon Peace, United States attorney for the Eastern District of New York, said in a statement on Monday that Ms. Gentili had been “tragically poisoned in her Brooklyn home from fentanyl-laced heroin.”

“Fentanyl is a public health crisis,” he added. “Our office will spare no effort in the pursuit of justice for the many New Yorkers who have lost loved ones due to this lethal drug.”

According to the indictment, Ms. Gentili left home for several hours on the evening of Feb. 5 and purchased drugs from Mr. Venti. When she returned home, she told her partner she was not feeling well and went to bed. The next morning, police officers found Ms. Gentili dead in her bedroom after her partner called 911.

Ms. Gentili’s family declined to comment on the indictments on Monday.

Prosecutors said she died as a result of the combined effects of heroin, xylazine, cocaine and fentanyl. They said an analysis of cellphone data and text messages showed that Mr. Venti sold the drug mixture to Ms. Gentili, and that he originally obtained the drugs from Mr. Kuilan.

Ms. Gentili was born in Argentina and came to the United States at age 26 to seek a better life. Instead, she found herself homeless, undocumented, addicted to heroin and trafficked for prostitution. After several arrests, she was jailed on Rikers Island. After her release, she spent 17 months in a rehab facility and said she remained sober for many years.

She was a lead plaintiff in a successful lawsuit against the Trump administration, which had tried to end protections for transgender patients under the Affordable Care Act, and she also lobbied for the repeal of a law that prohibited loitering for the purpose of prostitution, which activists said was used to target transgender women and women of color. She was lobbying to decriminalize sex work in New York at the time of her death.

Frank Tarentino, a special agent with the New York division of the Drug Enforcement Administration, said Ms. Gentili’s death was “a reminder of the dangers that illicit drugs have on all communities, including the L.G.B.T.Q.+ community.”

According to court documents, police officers searched the apartment in Williamsburg, Brooklyn, where Mr. Kuilan lived with his 85-year-old grandmother and found a handgun, ammunition and hundreds of small bags of fentanyl that contained approximately 30 grams of the drug.

Prosecutors said that was enough to administer lethal doses to several thousand people. A lethal dose for most adults is two milligrams, roughly the size of half a dozen grains of salt.

Mr. Kuilan and Mr. Venti were first arrested last month on a complaint that referred only to drug distribution. The indictment unsealed on Monday significantly raised the stakes, adding a charge of distribution of heroin and fentanyl causing death, which carries a sentence of 20 years to life in prison.

Mr. Kuilan’s mother, Elizabeth Pabellon, attended the arraignment on Monday along with his girlfriend, Denise Rivera, who wept. Ms. Pabellon said that the authorities broke down the door when they showed up to arrest her son on Monday morning.

“I’ve been doing everything to keep him on the straight and narrow,” Ms. Pabellon said after the hearing.

The indictment also charged Mr. Kuilan with being a felon in possession of a weapon. Prosecutors said in court filings that he had two prior felony convictions for drug possession and intent to sell, dating back to the early 2000s.

Mr. Kuilan had been confined to home detention on $100,000 bond. The judge raised that amount to $250,000 in light of the new charges; his grandmother is expected to use her co-op apartment as collateral.

Mr. Venti, who had been out on $100,000 bond signed by relatives, returned to Brooklyn federal court on his own. Prosecutors said that he also had two prior convictions in state court, for petty larceny and attempted drug sales. The prosecutor handling the case, Adam Amir, said that Mr. Venti had struggled with drug addiction, and suggested home detention and a curfew. But the judge allowed Mr. Venti, who his lawyer said works as an electrician, to remain free.

In a written statement to reporters, the lawyer, Joseph Turco, said Mr. Venti could not comment.

“But let me say that transgender issues have touched me personally and professionally,” Mr. Turco added. “I’ve been a fierce advocate. Cecilia Gentili will never be forgotten. We mourn her loss, and our hearts go out to her family.”

Fentanyl overdose is the leading cause of death for Americans ages 18 to 45, according to the D.E.A., which calls it “the greatest threat facing Americans today.” The Justice Department has targeted suppliers in China and Mexico, and federal prosecutors have sought to bring charges stemming from overdose deaths.

In one high-profile example, the drug dealers who supplied fentanyl-laced heroin to the heralded actor Michael K. Williams, known for playing Omar Little in “The Wire,” were charged in Manhattan federal court. Irvin Cartagena, the man who sold him the fatal dose, pleaded guilty to a narcotics conspiracy charge and was sentenced to 10 years in prison in August.